STACK 
INK  EX 


414 


Manual 

Training  Course 
in  Concrete 


CONCRETE  FOR 
PERMANENCE 


Oll  Qj  PUBLISHED  BY 

tity  Association  of 

figsxjj      American  Portland  Cement 
Manufacturers 

111  West  Washington  Street,  Chicago 
1916 


Lessons  and  General  Outline 
With  Suggested  Exercises 

FOR  A 

Manual  Training  Course 
in  Concrete 


Price,  25  Cents  per  Copy 


PUBLISHED  BY 

Association  of  American  Portland  Cement 

Manufacturers 

Chicago 


Copyrighted,  IQIS,  by  the  Association  of  American  Portland  Cement  Manufacturers 


I.  Manufacture  of  Portland  Cement 

CEMENTS  and  limes  have  been  used  since  the  dawn  of  civilization.  The 
famous  Appian  Way,  the  great  system  of  aqueducts  and  other  structures 
built  by  the  Romans,  are  to-day  in  a  remarkable  state  of  preservation,  and 
proof  positive  that  they  understood  the  use  of  cement  and  concrete. 

Notwithstanding  the  early  use  of  these  materials,  little  was  known  of 
their  chemistry,  and  no  substantial  advance  was  made  in  the  manufacture 
of  lime  and  cement  from  the  time  of  the  Romans  until  1756,  when  John 
Smeaton,  who  had  been  employed  by  the  English  government  to  build  a 
lighthouse  upon  a  group  of  gneiss  rocks  in  the  English  Channel,  near  the 
coast  of  Cornwall  in  Devonshire,  discovered  that  an  impure  or  clayey  lime- 
stone, when  burned  and  slaked,  would  harden  into  a  solid  mass  under  water, 
as  well  as  in  air.  This  discovery  of  Smeaton's  seemed  to  pave  the  way  for 
rapid  improvement  and  development  in  the  lime  and  cement  industries. 
In  1796  James  Parker,  of  Northfleet,  England,  obtained  a  patent  for  the 
manufacture  of  a  cement  which  he  aptly  named  Roman  cement.  Parker's 
process  consisted  of  burning  certain  stone  or  argillaceous  products  called 
" nodules  of  clay"  in  an  ordinary  lime  kiln,  and  then  grinding  to  a  powder. 
Cement  produced  in  this  manner  rapidly  gained  favor  among  engineers  and 
builders,  and  resulted  in  natural  cement  plants  springing  up  all  over  the 
continent  of  Europe,  England,  and  later,  about  1818,  in  the  United  States. 
In  1824  Joseph  Aspdin  took  out  a  patent  in  England  for  the  manufacture  of 
an  improved  cement,  produced  by  calcining  a  mixture  of  limestone  and  clay. 
To  the  resulting  powder  he  gave  the  name  of  "Portland  Cement,"  because, 
when  it  hardened,  a  yellowish-gray  mass  was  produced,  resembling  in 
appearance  the  stone  found  in  various  quarries  on  the  isle  of  Portland  off 
the  south  coast  of  England.  To  Joseph  Aspdin,  therefore,  is  given  the 
credit  of  making  the  first  Portland  cement,  and  he  is  generally  recognized  as 
the  father  of  the  modern  Portland  cement  industry. 

In  this  country  the  cement  industry  began  with  the  discovery,  in  1818, 
of  a  natural  cement  rock  near  Chittenango,  Madison  County,  New  York, 
by  Canvass  White,  an  engineer  on  the  Erie  Canal.  In  1825  cement  rock 
was  found  in  Ulster  County,  New  York,  along  the  Delaware  and  Hudson, 
and  in  1828  a  mill  was  built  in  Rosendale,  New  York,  and  it  was  from  this 
place  that  the  natural  cement  obtained  its  name.  Other  canals  along  which 

5 


cement  rock  was  discovered,  with  location  and  date,  are :  Illinois  &  Michi- 
gan Canal,  at  Utica,  111.,  in  1838;  James  River  Canal  at  Balcony  Falls,  Va., 
in  1848;  and  Lehigh  Coal  &  Navigation  Co.  Canal  at  Siegfried,  Penna.,  in 
1850. 

In  the  spring  of  1866  D.  O.  Saylor,  Esaias  Rehrig,  and  Adam  Woolever, 
all  of  Allentown,  Penna.,  formed  the  Coplay  Cement  Co.  and  located  near 
Allentown.  Mr.  Saylor  was  president  and  superintendent  of  the  mill. 
Early  in  the  seventies  he  began  experimenting  on  Portland  cement  from  the 
rock  in  the  quarries.  Noticing  that  although  the  harder  burned  portions  of 
his  Rosendale  clinker  gave  a  cement  which  in  a  short  period  of  time  would 
show  tensile  strength  equal  to  the  best  European  Portland  cement,  it  soon 
crumbled  away.  He  decided  that  this  was  because  the  raw  material  was 
not  properly  proportioned.  The  result  of  these  experiments  taught  him  to 
mix  his  high-limed  cement  rock  and  low-limed  cement  rock  in  the  correct 
proportions,  and  after  many  experiments  and  trials  true  Portland  cement 
was  produced  in  1875.  This  was  the  first  Portland  cement  made  in  the 
Lehigh  district  and  probably  in  the  United  States. 

Knowing  how  to  produce  a  high-grade  Portland  cement  was  not  all 
that  was  necessary.  The  next  difficulty  encountered  was  the  sale  of  it. 
This  new  American  Portland  cement  was  manufactured  at  a  high  cost; 
therefore  it  could  not  be  offered  at  a  lower  price  than  the  imported  article, 
and  it  was  only  by  aggressive  methods  that  Mr.  Saylor  secured  a  market  for 
his  product,  which  amounted  to  only  1700  barrels  a  year. 

About  the  same  time  Thomas  Millen  was  constructing  a  plant  at  South 
Bend,  Ind.,  and  plans  were  soon  under  way  at  Wampum,  Penna.,  Kala- 
mazoo,  Mich.,  and  Rockford,  Me.,  for  other  mills. 

This,  then,  was  the  small  beginning  of  the  American  Portland  cement 
industry,  which  has  grown  from  a  production  of  about  83,000  barrels  in 
1880,  and  less  than  1,000,000  in  1895,  to  the  total  of  approximately  92,000,- 
000  in  1913. 

This  brief  history  of  the  development  of  the  industry  prompts  the  ques- 
tion, "Just  what  is  Portland  cement?"  This  is  probably  answered  by  the 
definition  given  in  the  "Standard  Specifications  for  Portland  Cement" 
adopted  by  the  American  Society  for  Testing  Materials.  This  definition  is 
quite  formidable,  consisting,  as  it  does,  of  long  phrases  and  equally  long 
words,  and,  in  order  to  be  understood,  will  undoubtedly  require  some 
explanation. 

"Portland  cement  is  the  term  applied  to  the  finely  pulverized  product 
resulting  from  the  calcination  to  incipient  fusion  of  an  intimate  mixture  of 
properly  proportioned  argillaceous  and  calcareous  materials,  and  to  which 
no  addition  greater  than  3  per  cent,  has  been  made  subsequent  to  calcina- 
tion." 

6 


This  definition  gives,  in  addition  to  the  composition,  an  outline  of  the 
process  of  manufacture,  but  not  in  the  order  taken  in  manufacture.  These 
several  points  in  their  natural  order  would  be  as  follows: 

First — that  Portland  cement  is  composed  of  calcareous  and  argillaceous 
materials. 

Second — that  the  raw  materials  must  be  properly  selected  and  prepared 
for  the  process  of  manufacture,  due  regard  being  given  to  proper  proportion- 
ing. 

Third — that  there  must  be  an  intimate  mixture  of  the  raw  materials. 
This  necessitates  the  drying,  fine  grinding,  and  thorough  mixing  of  the  raw 
materials. 

Fourth — that  the  raw  material  or  mixture  must  be  burned  to  a  clinker  and 
that  the  heat  must  be  such  as  to  cause  a  partial  melting  of  the  ingredients. 

Fifth — that  the  finished  product,  or  Portland  cement,  is  the  product 
obtained  by  grinding  this  clinker  to  a  powder. 

The  last  clause  in  the  definition  provides  for  the  addition  of  a  small 
amount  of  some  material  to  regulate  the  setting  time,  but  limits  the  quan- 
tity to  prevent  adulteration. 

The  principal  elements  or  compounds  in  Portland  cement  are  lime 
(CaO),  silica  (SiO2),  and  alumina  (A12O3),  but  a  small  percentage  of  oxide  of 
iron  (Fe2O3)  and  magnesia  (MgO)  is  also  contained. 

The  composition  of  a  standard  Portland  cement  is  usually  within  the 
following  limits: 

COMPOUNDS  PER  CENT.  LIMITS 

Silica  (SiO,) 20  to  24 

Alumina  (A12O3) 5  to  10 

Iron  oxide  (F^Os) 2  to    5 

Lime  (CaO) 60  to  65 

Magnesia  (MgO) 1  to    4 

Sulphur  trioxide  (SO3)  . : 0.5  to    1.75 

Nature  has  provided  an  abundance  of  these  calcareous  and  argillaceous 
materials  suitable  for  the  manufacture  of  Portland  cement.  The  calcare- 
ous variety  is  always  in  the  form  of  calcium  carbonate,  such  as  limestone, 
chalk,  marl,  or  the  precipitated  form  obtained  as  a  waste-product  from  the 
manufacture  of  alkalis.  The  argillaceous  division  includes  clay,  shale  and 
slate,  cement  rock,  and  selected  blast  furnace  slag.  Cement  is  made  in  this 
.country  from  all  these  materials,  each  plant  using  one  of  the  calcareous 
combined  with  one  of  the  argillaceous  materials. 

Portland  cement  may  also  be  divided  into  classes,  according  to  the 
method  of  manufacture,  which  are  as  follows : 

1.  Wet  process. 

2.  Semi-wet  process. 

3.  Dry  process. 

7 


In  the  wet  process  the  raw  materials  are  intimately  mixed,  ground,  and 
fed  (in  the  form  of  a  slurry  containing  sufficient  water  to  make  it  of  a  fluid 
consistence)  into  the  rotary  kilns.  In  the  semi-wet  process  a  similar  but 
drier  slurry  is  used,  while  in  the  dry  process  raw  materials  are  ground, 
mixed,  and  burned  in  the  dry  state. 

Because  of  the  fact  that  the  larger  portion  of  Portland  cement  manu- 
factured in  the  United  States  to-day  is  made  by  plants  using  the  dry  proc- 
ess, the  description  of  the  process  of  manufacture  will  be  confined  to  an 
account  of  this  method. 

The  manufacture  of  Portland  cement  itself  is  divided  into  five  heads, 
as  follows : 

1.  Mining  and  quarrying  of  raw  materials. 

2.  Drying  and  grinding. 

3.  Proportioning  and  mixing. 

4.  Burning  the  mixed  materials  to  incipient  fusion. 

5.  Grinding  the  clinker  thus  burned  to  an  extremely  fine  powder,  mean- 
while adding  the  proper  proportion  of  gypsum,  the  resulting  powder  being 
known  as  Portland  cement. 

The  excavation  of  the  raw  materials  is  the  first  step  toward  the  actual 
manufacture  of  Portland  cement,  and  the  one  concerning  which  least  has 
been  published.  Local  conditions  enter  into  this  preliminary  stage  to  such 
an  extent  that  few  general  statements  can  be  made  concerning  it.  The 
natural  raw  materials  are  worked  by  one  of  three  general  methods.  First, 
quarrying  and  digging  from  open  pits.  Second,  mining  from  underground 
workings.  Third,  dredging  from  deposits  covered  by  water.  Inasmuch 
as  this  paper  deals  with  the  manufacture  of  Portland  cement  itself,  we  will 
not  go  deeply  into  the  preparation  of  the  raw  material. 

The  raw  materials  for  the  greater  part  of  the  Portland  cement  made  in 
the  United  States  to-day  are : 

(a)  Limestone  and  clay,  shale  or  slag. 

(6)  Cement  rock  and  pure  limestone. 

The  method  of  quarrying  the  rocks  usually  follows  that  customary  in  all 
quarry  operations.  The  rock  is  dislodged  from  the  quarry  face  by  means  of 
an  explosive  and  then  loaded  into  side  dump  cars  or  aerial  trams  by  either 
steam  shovel  or  manual  labor,  preferably  the  former.  The  stone  is  then 
conveyed  to  the  stone  house,  where  it  is  crushed  to  comparatively  small 
sizes  and  then  transported  to  storage-bins  before  being  mixed  with  the 
other  ingredients.  While  in  storage  the  stone  may  be  sampled  and  ana- 
lyzed. Another  method  used  is  to  pass  the  limestone,  shale,  or  cement  rock 
through  crushers  and  ball  mills,  or  other  preliminary  grinders,  from  which 
it  is  conveyed  to  storage-bins.  The  ball  mills  are  cylindrical  steel  drums 
containing  a  quantity  of  steel  balls.  The  material  to  be  ground,  after  dry- 


ing,  is  continuously  added.  As  the  cylinder  rotates  the  balls  roll,  thus 
grinding  the  rocks  to  coarse  grits.  The  coarse  grits  are  then  run  into  stor- 
age-bins. Tube  mills  which  are  used  further  on  in  the  process  are  similar 
in  general  to  the  ball  mills. 

Shale,  which  for  practical  purposes  may  be  looked  upon  as  solidified 
clay,  is  excavated,  dried,  ground,  and  then  conveyed  to  storage-bins. 

After  the  raw  materials  have  been  drawn  from  their  respective  bins  and 
accurately  proportioned  by  weighing,  they  are  delivered  to  a  screw  con- 
veyor which  completes  the  mixing  and  delivers  the  combined  material  to 
the  tube  mills.  The  tube  mills  are  revolving  cylinders  half  full  of  flint 
pebbles  or  steel  slugs  which  reduce  the  material  continually  being  fed 
into  practically  the  fineness  of  finished  cement.  At  this  point,  however, 
we  are  a  long  way  from  the  finished  cement,  since  the  product  of  these 
mills  has  a  long  journey  before  it  is  ready  for  the  consumer. 

All  the  tube  mills  deliver  to  the  same  conveyor,  which  results  in  a  uni- 
form product  of  the  raw  material  mill  as  a  whole.  At  frequent  intervals 
samples  are  taken  from  the  belt  and  delivered  to  a  sample  can  which  is 
collected  at  stated  intervals  by  one  of  the  chemist's  assistants.  The  sample 
is  then  taken  to  the  laboratory  where  tests  are  made  to  determine  its 
composition. 

From  the  tube  mills  the  material  is  fed  to  the  kilns  through  a  system  of 
conveyors.  The  kilns  themselves  are  from  6  to  8  feet  in  diameter  and 
from  60  to  150  feet  long,  125  feet  being  the  average  length.  They  are 
lined  with  fire  brick  and  revolve  at  about  the  rate  of  one  revolution  per 
minute.  It  is  estimated  that  a  particle  of  raw  material  takes  about  an 
hour  to  traverse  the  entire  distance  from  the  feed  to  the  outlet.  Powdered 
bituminous  coal,  crude  oil,  or  gas  is  used  as  fuel,  powdered  coal  being 
the  one  most  generally  used.  It  is  blown  into  the  kiln  at  the  end  opposite 
that  at  which  the  raw  materials  enter. 

The  raw  material  entering  as  a  powder  is  gradually  brought  to  the  point 
of  incipient  fusion  at  a  temperature  of  2500°  to  3000°  Fahrenheit,  producing 
clinkers  varying  in  size  from  ^  inch  up  to  about  1^  inches  in  diameter.  It 
may  also  be  mentioned  that  the  clinker  is  red  hot  when  discharged,  but  is 
soon  cooled  by  sprays  of  water  or  cold-air  blasts  which  are  played  over  the 
elevator  and  also  upon  the  clinker  when  delivered  to  the  storage  piles. 

If  we  go  to  the  front  of  the  kilns,  we  will  see  that  the  method  of  heating 
is  the  same  as  used  in  drying  slag  and  limestone,  with  the  difference  that 
considerably  more  fuel  and  blast  are  required  as  a  higher  temperature  is 
necessary.  By  means  of  smoked  glasses  experienced  workmen  are  able  at 
all  times  to  note  the  condition  within  the  kiln. 

From  the  kiln,  the  clinker  may  go — (a)  to  the  clinker  storage  pile  for 
later  grinding,  or  (6)  directly  to  the  grinding  department. 

Either  before  or  after  the  preliminary  grinding  of  the  clinker  by  jaw 

9 


crusher  it  is  usual  to  add  gypsum,  either  by  hand  or  automatically,  in 
order  to  retard  the  setting  time  of  the  cement.  Were  gypsum  not  added, 
the  cement  would  harden  quickly  and  develop  little  strength.  Approxi- 
mately two  pounds  of  gypsum  are  added  to  every  100  pounds  of  clinker. 
This  is  controlled  by  the  chemist  from  analyses  of  the  finished  cement  and 
from  the  setting  determinations  made  hourly  in  the  physical  laboratory. 

After  the  gypsum  has  been  added,  the  material  is  delivered  to  the  tube 
mills,  which  complete  the  grinding.  These  tube  mills  are  similar  to  those 
which  grind  the  raw  material,  and  are  also  half  full  of  flint  pebbles  or  steel 
slugs  which  rotate  and  grind  one  against  the  other,  reducing  the  cement  to 
the  fineness  with  which  every  one  is  familiar.  There  are  also  special  grind- 
ing machines  which  supplement  the  tube  mills.  Frequent  samples  are 
taken  of  the  finished  product.  It  is  customary  to  make  up  test  pats  every 
hour  and  briquettes  twice  a  day,  while  in  the  chemical  laboratory  complete 
analyses  are  continually  being  made  of  the  finished  product,  as  well  as  of 
the  raw  materials  which  enter  into  its  composition. 

The  bins  used  for  storage  are  similar  to  those  used  in  storing  grain.  The 
material  is  deposited  in  these  bins  by  means  of  a  conveyor  belt  with  a  trip- 
ping mechanism  which  can  be  run  from  one  end  of  the  house  to  the  other. 
The  cement  is  drawn  out  from  below  through  holes  in  the  floor,  delivering  to 
screw  conveyors  underneath.  A  peculiarity  noted  in  drawing  cement  or 
other  similar  material  is  that  when  the  drawing  is  started,  the  cement  comes 
out  as  an  average  of  the  entire  bin.  If,  for  example,  only  one  hole  is  opened 
below  the  bin,  the  top  surface  will  "cave"  slightly  after  the  drawing  is  con- 
tinued for  a  short  time,  and  from  then  on  small  portions  are  observed  to 
fold  in  from  the  outside  of  the  crater,  indicating  that  an  average  of  the  entire 
stock  is  being  delivered  below.  From  each  of  the  tunnels  conveyors  carry 
the  cement  to  large  elevators,  which  raise  it  finally  to  large  hoppers  above 
the  packing  floor,  on  which  are  installed  packing  machines.  A  cement 
sack  may  be  filled  (when  packing  machines  are  used)  through  the  bottom, 
and  not  through  the  top.  The  bags  are  tied  previous  to  filling,  and  a 
valve  may  be  discovered  by  careful  inspection  of  any  of  the  standard  sacks 
used  to-day.  Every  modern  cement  manufacturer  employs  a  packing 
machine  for  filling  sacks.  As  sacks  are  filled  a  conveyor  belt  running  the 
entire  length  of  the  packing  room  unloads  them  within  a  few  feet  of  the 
car,  ready  for  shipment. 

REFERENCES 

"Portland  Cement,"  by  Richard  R.  Meade.     Published  by  The  Chemical  Publishing 

Co.,  Easton,  Pa. 
"Materials  of  Construction,"  by  J.  B.  Johnson.     Published  by  John  Wiley  &  Sons, 

New  York  City. 

10 


II.  Concrete  Aggregates 

PRESENT-DAY  success  in  the  use  of  concrete  is  not  due  to  any  particular 
"discovery,"  but  is  the  result  of  a  consistent,  scientific  study  and  investiga- 
tion of  the  component  materials. 

As  ordinarily  employed,  the  term  "aggregates"  includes  not  only  gravel 
or  stone,  the  coarse  material  used,  but  also  the  sand,  or  fine  material,  mixed 
with  the  cement  to  form  either  mortar  or  concrete.  Fine  aggregate  is 
defined  as  any  suitable  material  that  will  pass  a  No.  4  sieve  or  screen  having 
four  meshes  to  the  linear  inch,  and  includes  sand,  stone  screenings,  crushed 
slag,  etc.  By  coarse  aggregate  is  meant  any  suitable  material,  such  as 
crushed  stone  and  gravel,  that  is  retained  on  a  No.  4  sieve.  The  maximum 
size  of  coarse  aggregate  depends  on  the  class  of  structure  for  which  the  con- 
crete is  to  be  used. 

The  strength  of  concrete  can  never  be  greater  than  that  of  the  materials 
used  as  aggregates.  Nothing  is  more  conducive  to  unsatisfactory  results  in 
concrete  work  than  poor  aggregates.  The  condition  of  the  cement,  methods 
of  mixing,  the  proportions  used,  and  the  amount  of  water  added,  also  the 
method  of  depositing  and  curing  concrete,  all  have  their  effect  upon  its 
density  and  strength,  but  even  with  the  most  careful  attention  given  to 
these  details,  good  results  are  impossible  without  good  aggregates. 

The  fact  that  the  aggregates  seem  of  good  quality  yet  may  be  proved 
totally  unsuitable  shows  that  study  and  careful  tests  are  necessary  if  the  best 
results  are  to  be  obtained.  The  idea  that  the  strength  of  concrete  depends 
entirely  upon  the  cement  and  that  only  a  superficial  examination  of  aggre- 
gates is  necessary,  is  altogether  too  prevalent.  The  man  who  understands 
his  aggregates,  grades  them  properly,  sees  that  they  are  washed,  if  neces- 
sary, then  mixes  them  in  proportions  determined  by  thorough  testing,  study, 
or  actual  experiences,  is  the  one  who  will  make  the  best  concrete. 

In  the  selection  and  use  of  sand  more  precautions  are  necessary  than 
for  the  coarser  aggregate,  due  to  its  physical  condition  and  a  wider  variation 
in  properties.  A  knowledge  of  these  properties  and  of  the  method  of  analy- 
sis to  determine  the  suitability  of  sand  for  use  in  mortar  and  concrete  may 
be  easily  applied  to  an  analysis  of  the  coarse  aggregate.  (Stone  screenings, 
broken  stone,  and  gravel  will  be  discussed  only  where  their  properties  and 
the  methods  of  examining  them  differ  from  those  of  sand.) 

11 


Origin  and  Composition 

Geologists  classify  rock  in  one  of  two  large  groups: 

1.  Igneous. 

2.  Sedimentary. 

Igneous  Rocks 

Igneous  rocks  are  those  which  have  been  formed  by  the  cooling  of  fused 
material.  They  are  classified  either  as  massive  or  laminated,  according  to 
their  structure.  The  massive  igneous  rocks  are  those  which  have  been 
solidified,  undisturbed,  from  a  fused  state,  and  which  have  not  been  subse- 
quently subjected  to  severe  external  stresses.  When  the  rock  was  sub- 
jected, either  during  or  after  cooling,  to  external  pressure,  a  laminated 
structure  seems  to  have  resulted,  with  the  component  minerals  arranged 
in  more  or  less  definite  alternating  bands.  Most  granites  and  all  trap-rock 
belong  to  the  first  class,  while  rocks  of  the  second  class  are  termed  gneisses. 

Sedimentary  Rocks 

Sedimentary  rocks  are  those  derived  from  the  breaking  up  or  disinte- 
gration of  preexisting  strata,  the  material  so  obtained  being  carried,  usually 
in  suspension  or  solution,  to  some  point  where  it  is  redeposited  as  a  bed  of 
fine  particles,  clay,  or  ealcareous  material,  such  as  shells,  marls,  etc.  Sub- 
sequently, this  loosely  deposited  material  may  become  consolidated  and 
compacted  by  pressure  or  other  agencies,  the  result  being  the  formation  of 
sandstone,  shale  and  slate,  or  limestones,  dolomites,  and  marbles. 

Sedimentary  rocks  may  be  classified  on  a  combined  chemical  and  physical 
basis,  distinguished  by  the  material  of  which  they  are  chiefly  composed,  as : 

1.  Silicious  sedimentary  or  sandstone  and  conglomerates. 

2.  Argillaceous  or  clayey  rocks,  such  as  shales  and  slates. 

3.  Calcareous  rocks,  namely,  marble,  compact  limestone,  granular 

limestone,  and  magnesian  limestone,  or  dolomites. 

Sources  of  Supply 

The  materials  commonly  used  as  coarse  concrete  aggregate  in  different 
places  throughout  the  United  States  are  the  sedimentary  rocks,  which  may 
be  grouped  into  three  classes  on  the  basis  of  origin  : 

1.  Glacial  deposits. 

2.  Coastal  plain  deposits. 

3.  Stream  deposits. 

All  these  deposits  contain  more  or  less  silt,  clay,  loam,  or  other  finely 
divided  impurities. 

The  gravel  beds  of  the  glacial  drifts  furnish  excellent  material  for  con- 

12 


crete.  Baker,  in  "Roads  and  Pavements,"  says:  "Glacial  gravel  exists  in 
considerable  quantities  in  western  Pennsylvania,  in  the  greater  part  of 
Ohio,  in  northern  Indiana,  and  in  Illinois,  and  to  some  extent  in  several  of 
the  States  of  the  southwest.  There  are  large  areas  of  this  gravel  in  Wiscon- 
sin, Minnesota,  and  Iowa." 

* 

Physical  Conditions 

Sands  differ  not  only  in  chemical  and  mineralogic  composition,  but  in 
physical  condition.  They  often  contain  many  impurities,  and  the  methods 
for  determining  their  presence,  as  well  as  their  effects,  should  be  known. 

Impurities 

Many  of  these  impurities  impair  the  hardening  properties  of  cement, 
and  hence  the  strength  of  the  concrete  in  which  used.  Much  has  been 
written  relative  to  the  effect  of  clay  upon  concrete,  and  many  contradictory 
opinions  have  been  advanced.  Engineers  are,  however,  fairly  in  accord  on 
certain  conclusions.  When  clay  exists  as  a  coating  on  the  particles  of  sand 
aggregate,  it  is  undoubtedly  injurious,  as  proper  adhesion  between  the 
cement  and  the  sand  surfaces  is  then  prevented.  When,  however,  clay  of  a 
silicious  nature  in  the  form  of  separate  particles  exists  to  a  small  extent 
throughout  the  mass  of  aggregate,  it  appears  to  cause  no  serious  harm  in 
many  kinds  of  concrete  work.  Clay  in  this  form  acts  as  an  adulterant, 
without  seriously  reducing  the  tensile  strength  of  the  concrete.  Their 
opinions,  however,  are  based  largely  on  the  results  of  tensile-strength  tests 
on  relatively  dry  mixtures.  It  is  doubtful  whether,  under  field  conditions, 
or  even  in  large  compression-test  specimens  made  up  in  the  laboratory, 
these  results  would  be  obtained.  An  excess  of  clay  tends  to  lead  one  into 
believing  that  the  concrete  contains  an  excess  of  cement  rather  than  a 
shortage.  The  only  advantage  that  can  be  claimed  for  the  presence  of  clay 
is  that  it  increases  the  density  of  the  concrete  by  filling  some  of  the  voids. 

The  presence  of  clay  in  sand  may  be  detected  by  the  well-known  method 
of  rubbing  the  material  between  the  hands.  If  clean,  the  sand  should  not 
adhere  to  or  discolor  the  hands.  Also  a  small  quantity  of  the  sand  may  be 
stirred  or  shaken  in  a  tumbler  or  bottle  of  water,  when  the  presence  of  clay 
will  at  once  be  shown  by  a  characteristic  cloudiness  of  the  water.  Since  the 
clay  remains  longer  in  suspension  than  the  sand,  it  will  separate  and  settle 
later  in  a  layer  on  top. 

Vegetable  Matter 

A  coating  of  vegetable  matter  on  sand  grains  appears  not  only  to  prevent 
the  cement  from  adhering,  but  to  affect  it  chemically.  Frequently  a  quan- 
tity of  vegetable  matter  so  small  that  it  cannot  be  detected  by  the  eye,  and 

13 


only  slightly  disclosed  in  chemical  tests,  may  prevent  the  mortar  from  reach- 
ing any  appreciable  strength.  Concrete  made  with  such  sand  usually 
hardens  so  slowly  that  the  results  are  questionable  and  its  use  is  prohibited. 
Other  impurities,  such  as  acids,  alkalis,  or  oils  in  the  sand  or  mixing  water, 
usually  make  trouble. 

The  usual  way  of  determining  the  quality  of  sand  is  to  make  up  bri- 
quettes in  the  proportions  of  one  part  cement  to  three  parts  of  the  sand  to 
be  tested,  and  compare  the  results  with  the  strength  of  a  mortar  made  with 
the  same  cement  and  standard  Ottawa  sand  in  like  proportions  and  of 
standard  consistence.  Standard  Ottawa  sand  is  exceptionally  uniform,  and 
is  obtained  from  Ottawa,  111. 

The  presence  of  moisture  in  sand  may  make  proper  mixing  with  other 
materials  somewhat  difficult,  as  a  uniform  distribution  of  cement  in  the 
mortar  is  hard  to  obtain. 

General  Requirements 

The  quality  of  concrete  is  affected  by — 

1.  The  hardness,  or  crushing  strength,  of  the  aggregates. 

2.  Their  durability  or  resistance  to  weather  and  abrasion. 

3.  Grading,  or  maximum  and  relative  sizes,  of  particles. 

4.  Cleanliness  or  freedom  from  foreign  materials. 

5.  The  shape  and  nature  of  the  surface  of  the  particles. 

Hardness 

The  hardness  of  the  material  grows  in  importance  with  the  age  of  con- 
crete. Because  of  the  rounded  surface  of  the  aggregate,  gravel  concrete 
one  month  old  may  be  weaker  than  concrete  made  with  comparatively  soft 
broken  stones;  but  when  one  year  old,  it  may  surpass  in  strength  the  broken- 
stone  concrete,  because,  as  the  cement  becomes  harder  and  the  bond  firmer, 
the  resistance  of  the  aggregate  to  stress  becomes  a  more  important  factor. 

The  grains  should  offer  at  least  as  high  a  resistance  to  crushing  as  does 
the  cement  after  attaining  its  maximum  strength.  In  comparing  sands  of 
the  same  kind,  those  having  the  highest  specific  gravity  are  likely,  as  a  rule, 
to  be  the  strongest.  This  applies  in  a  general  way  to  the  comparison  of 
different  kinds  of  rock  also. 

Value  of  Different  Rocks 

Different  rocks  of  the  same  class  vary  so  widely  in  texture  and  strength 
that  it  is  impossible  to  give  definitety  their  relative  values  as  aggregate. 
However,  a  comparison  of  a  large  number  of  tests  of  concrete  made  with 
broken  stone  from  different  kinds  of  rock  material  indicates  that  its  value 

14 


as  an  aggregate  is  largely  governed  by  the  actual  strength  of  the  stone  itself, 
the  strongest  stone  producing  the  strongest  concrete. 

Comparative  tests  discussed  by  various  writers  indicate  that,  in  the 
order  of  their  value  for  concrete,  the  different  materials  stand  approximately 
as  follows : 

1.  Granite. 

2.  Trap-rock. 

3.  Gravel. 

4.  Marble. 

5.  Limestone. 

6.  Slag. 

7.  Sandstone. 

The  grading,  that  is,  the  relative  size  and  quantity  of  the  particles  of  an 
aggregate,  determines  in  a  large  measure  the  density  of  the  mass,  which 
greatly  affects  the  quality  of  the  concrete.  A  coarse,  well-graded  aggre- 
gate produces  a  denser  and  stronger  concrete  or  mortar.  A  sufficient  quan- 
tity of  fine  grains  is  valuable  in  grading  the  material  and  reducing  the  voids, 
but  an  excess  has  a  tendency  to  diminish  the  compression  strength  con- 
siderably. 

Weights  and  Voids 

A  high  unit  weight  of  material  and  a  corresponding  low  percentage  of 
voids  are  indications  of  coarseness  and  good  grading  of  particles.  However, 
the  impossibility  of  establishing  uniformity  of  weight  and  measurement 
due  to  different  percentages  of  moisture  and  different  methods  of  handling 
makes  these  results  merely  general  guides  that  seldom  can  be  taken  as 
positive  indications  of  true  relative  values.  This  is  especially  true  of  the 
fine  aggregates  in  which  percentages  of  voids  increase  and  weights  decrease 
with  the  addition  of  moisture  up  to  about  6  per  cent. 

Maximum  Size 

Within  reasonable  limits  the  strength  of  concrete  increases  with  the  size 
of  stones.  For  mass  concrete  the  practical  maximum  size  is  2^  or  3  inches. 
In  thin  reinforced  structures,  such  as  floors  and  walls,  the  size  must  be  such 
as  can  be  worked  readily  about  the  reinforcing  metal,  and  1%  inches  is 
generally  the  maximum. 

Cleanliness 

As  stated,  the  particles  of  rock  should  be  free  from  dirt  and  dust,  and 
should  not  be  used  when  even  partly  covered  with  clay;  such  impurities 
prevent  the  cement  from  obtaining  a  bond  on  the  surface  of  the  particles, 
and  often  contain  materials  which  retard  the  hardening  of  the  mortar  or 

15 


concrete  and  prevent  it  from  acquiring  normal  strength  within  a  reasonable 
length  of  time. 

An  excess  of  clay  or  dirt  in  any  form  also  affects  the  color  of  the  con- 
crete when  hardened,  and  necessitates  more  thorough  mixing. 

Shape  of  Particles 

The  shape  of  the  rock  particles  influences  the  strength  of  the  mortar  or 
concrete.  Flat  particles  pack  loosely  and  generally  are  inferior  to  those  of 
cubical  fracture. 

Analysis 

The  chief  value  of  an  analysis  of  any  sand  results  from  the  comparison 
of  its  various  properties  with  those  of  other  sand  tested  under  similar  con- 
ditions and  recognized  as  of  a  good  quality. 

Classification 

The  sands  in  common  use  as  aggregate  throughout  the  United  States 
are  sedimentary,  hence  the  classification  can  usually  be  confined  to  the 
degree  of  consolidation  and  the  kind  of  material,  on  the  basis  of  whether 
its  formation  is  chiefly  siliceous  or  calcareous.  Hardness  and  texture  are 
ready  aids  in  these  determinations,  which  may  be  conducted  in  an  elemen- 
tary manner. 

The  natural  sands  are  usually  siliceous,  but  they  vary  in  degree  of  con- 
solidation, which  determines  in  a  large  measure  the  crushing  strength  and 
durability  of  the  concrete.  Their  durability  is  also  dependent  upon  the 
nature  and  amount  of  impurities  present,  as  feldspar,  mica,  oxides  of  metals, 
etc.  Such  impurities  account  largely  for  the  variegated  coloring  in  sand 
grains. 

Specific  Gravity 

As  sands  or  rocks  of  the  same  kind  and  having  the  highest  specific 
gravity  are  likely  to  be  strongest,  a  determination  of  the  specific  gravity  of 
different  sands  is  valuable,  since  it  is  a  ready  indication  of  the  nature  and 
hardness  of  the  material.  As  a  rule,  sand  having  the  highest  specific  grav- 
ity, other  things  being  equal,  will  give  the  best  results. 

The  specific  gravity  of  a  material  is  determined  by  dividing  its  weight 
by  the  weight  of  the  water  which  it  displaces  when  immersed.  Take  a 
convenient  amount  of  sand,  screen  it  through  a  %-mch  screen,  dry,  and 
weigh.  Then  place  some  water  in  a  glass  graduate,  read  the  height  of  the 
water,  add  the  sand,  and  again  read  the  height  of  the  water.  The  differ- 
ence in  readings  will  be  the  amount  of  water  displaced  by  the  sand.  Divide 
the  weight  of  this  water  into  the  weight  of  the  sample  of  sand.  The  result 

16 


will  be  the  specific  gravity  of  the  sand.  (For  detailed  methods  of  testing 
sand  and  cement-sand  mortar,  Bulletin  No.  33,  of  the  United  States  Bureau 
of  Standards,  should  be  referred  to.) 

Determinations  Necessary 

Physical  Analysis. — The  determinations  necessary  for  a  good  physical 
analysis  of  sand  are: 

1.  Strength  and  density  in  mortar. 

2.  Gradation  and  effective  size  of  grains. 

3.  Cleanliness,  including  per  cent,  and  nature  of  silt. 

4.  Percentage  of  voids. 

Density 

In  the  study  of  sands,  a  determination  of  their  density  is  important  as 
regards  both  quality  and  economy.  Other  physical  conditions  being  equal, 
the  sand  which  produces  the  smallest  volume  of  plastic  mortar  when  mixed 
with  cement  in  the  required  proportions  makes  the  strongest  and  least  per- 
meable mortar,  and  the  densest  mortar  will  be  the  strongest.  This  requires 
that  the  sand  be  graded  from  coarse  to  fine,  the  coarser  particles  predominat- 
ing. (The  question  of  determining  density  will  be  discussed  in  the  lesson 
on  Proportioning,  Mixing,  and  Placing.) 

Gradation  and  Effective  Size 

Sand  in  which  coarse  grains  predominate  will  produce  a  greater  strength 
in  mortar  than  that  made  up  of  fine  grains,  because  it  presents  a  more  com- 
pact mass,  as  well  as  a  smaller  amount  of  surface  area  to  cover  with  cement, 
and  usually  a  smaller  percentage  of  voids.  A  fine  sand  requires  more  thor- 
ough mixing  than  coarse  sand  in  order  to  get  a  proper  distribution  of  cement. 

The  size  of  sand  grains  is  so  important  that  it  is  often  profitable  to  ship 
a  coarse  sand  a  considerable  distance  rather  than  use  a  local  fine  sand. 
Feret,  the  French  authority,  computed  that  it  was  more  economical  to  use 
coarse  instead  of  fine  sand,  even  though  the  cost  is  several  times  as  great. 
It  does  not  follow,  however,  that  because  coarse  particles  have  the  smallest 
area  per  unit  of  volume,  the  aggregate  should  all  be  large.  Particles  of  the 
same  size  form  a  volume  having  a  larger  percentage  of  voids  than  if  graded 
in  size,  hence  requiring  a  larger  proportion  of  cement  to  produce  the  maxi- 
mum strength. 

Granulometric  Composition 

The  determination  of  the  granulometric  composition  or  mechanical 
analysis  of  sand  is  made  in  order  to  study  its  properties  and  to  judge  of  its 
value  compared  with  other  sands,  and,  if  necessary,  regrade  its  grains  so 
that  a  denser  mass  may  be  secured. 

2  17 


That  the  strength,  quality,  and  value  of  a  sand  may  be  indicated  by 
ascertaining  whether  the  majority  of  its  particles  are  coarse,  medium,  or 
fine  has  been  generally  established,  and  it  is  also  important  to  determine 
the  relative  degree  of  coarseness  and  fineness. 

The  percentages  of  different  size  grains  are  frequently  determined  by  a 
mechanical  analysis.  The  sample  is  first  screened  through  a  number  of 
sieves  of  successive  sizes,  and  the  percentage  by  weight  retained  on  each 
recorded. 

For  this  work  the  following  sieves  are  recommended : 

COMMERCIAL  No.  OF  SIEVE 

4 

10 

20 

30 

40 

50 

80 
100 
200 

A  standard  sieve  is  made  of  woven  brass  wire,  set  into  a  hard  brass 
frame,  8  inches  in  diameter  and  2^  inches  deep.  These  sieves  are  described 
by  numbers  corresponding  approximately  to  the  number  of  meshes  per 
linear  inch. 

All  material  referred  to  as  sand  must  pass  a  No.  4  sieve.  Not  more  than 
20  per  cent,  should  pass  a  sieve  having  50  meshes  per  linear  inch,  and  not 
more  than  5  per  cent,  should  pass  a  sieve  having  100  meshes  per  linear  inch. 

The  tabulated  results  showing  the  percentages  by  weight  retained  on  the 
different  sieves  form  a  valuable  basis  for  a  study  of  the  effective  sizes  of 
grains,  and  for  comparison  with  other  sands  whose  value  in  mortar  or  con- 
crete has  already  been  determined. 

Cleanliness 

The  effect  of  dirty  sand  is  dependent  upon  the  quantity  and  nature  of 
the  impurities  and  the  form  and  manner  in  which  they  are  present.  The 
manner  in  which  silt  is  contained  in  sand  may  be  determined  by  inspection. 
The  silt  in  a  sand  is  that  material  which  in  solution  and  in  suspension  is 
carried  away  in  wash-water  so  applied  as  not  to  remove  the  small  grains  of 
sand.  This  amount  may  be  ascertained  by  determining  either  the  amount 
of  substance  contained  in  the  wash-water  or  the  amount  of  loss  sustained  by 
the  sand  through  washing.  The  latter  method  is  more  generally  used. 

18 


If  the  silt  is  vegetable  matter  in  a  gelatinous  or  viscous  state,  forming  a 
colloidal  covering  over  the  surface  of  the  sand  grains,  its  presence  may  be 
determined  by  immersing  the  material  in  a  dilute  solution  of  sulphuric  or 
hydrochloric  acid  and  comparing  the  strength  of  cement  mortar  made  from 
the  sand  before  immersion  and  after  the  sand  has  been  treated  with  the 
dilute  acid  and  thoroughly  cleansed  by  washing. 

Voids 

Voids  are  air-spaces  between  the  grains  and  are  usually  referred  to  as  a 
percentage  of  the  whole.  A  sand  consisting  of  grains  all  uniform  in  size  will 
present  the  maximum  of  voids.  This  can  be  illustrated  as  follows:  Perfect 
spheres  of  equal  size  piled  in  the  most  compact  manner  leave,  theoretically, 
but  26  per  cent,  of  voids.  The  only  requirement  is  that  the  spheres  be  of 
equal  size.  Suppose,  now,  that  the  spaces  between  such  a  pile  of  equal- 
sized  spheres  were  filled  with  other  perfect  spheres  of  diameter  just  sufficient 
to  touch  the  larger  spheres,  the  voids  in  the  total  included  mass  would  be 
reduced  theoretically  to  20  per  cent. ;  and  should  this  be  followed  up  with 
smaller  spheres,  the  air-spaces  or  voids  could  be  reduced  sufficiently  to  make 
the  mass  water-tight.  Practically,  however,  a  mass  of  equal-sized  spheres 
will  be  found  by  experiment  to  contain  about  44  per  cent,  voids,  which  may 
be  reduced  as  indicated  above.  The  shape  of  the  particles  also  affects  the 
percentage  of  voids.  Round  particles  compact  more  readily  and  firmly 
and  with  less  difficulty  than  angular  particles. 

Conclusion 

The  scope  of  concrete  work  has  become  so  great  that  it  demands  a 
nation-wide  study  of  aggregates.  But  such  study  alone  will  not  solve  all 
the  problems  and  insure  good  work  in  the  future.  It  will,  however,  serve  to 
give  an  idea  of  the  relative  merits  of  the  various  aggregates  available.  We 
now  have  standard  specifications  which  demand  certain  requirements  from 
the  cement  manufacturers.  How  much  more  do  we  need  standard  speci- 
fications for  the  selection  of  concrete  aggregates?  The  preceding  para- 
graphs have,  in  a  brief  way,  given  you  some  idea  of  the  properties  required 
in  good  aggregate,  which  are,  briefly,  good  grading,  cleanliness,  and  dura- 
bility. Therefore,  with  good  aggregates,  standard  Portland  cement,  and 
careful  and  efficient  workmanship,  good  concrete  can  easily  be  obtained. 

REFERENCES 

"Concrete,  Plain  and  Reinforced,"  by  Taylor  and  Thompson.     Published  by  John 

Wiley  and  Sons,  New  York. 
"Materials  of  Construction,"  by  J.  B.  Johnson.     Published  by  John  Wiley  and  Sons, 

New  York. 
"Engineering  Geology,"  by  Ries  and  Watson.     Published  by  John  Wiley  and  Sons, 

New  York. 

19 


III.  Proportioning,  Mixing,  and  Placing  of 
Concrete 

I.  Proportioning 

Theory 

In  order  to  comprehend  the  importance  of  correctly  proportioning  the 
ingredients  used  in  the  making  of  concrete  we  must  in  the  beginning  obtain 
a  correct  idea  of  the  theory  of  the  material  we  propose  to  manufacture. 

The  aggregates  consisting  of  sand  and  gravel  or  broken  stone  are  wholly 
inert  until  combined  with  Portland  cement.  Consequently  it  is  of  prime 
importance  that  every  piece  of  coarse  aggregate  be  thoroughly  surrounded 
with  sand-cement  mortar  and  that  every  grain  of  sand  be  inclosed  in  a  film 
of  neat  cement.  In  so  far  as  actual  practice  departs  from  this  fundamental 
principle,  just  so  far  will  the  bonding  be  defective. 

The  second  important  principle  of  concrete  composition  is  that  voids 
shall  be  eliminated  by  such  gradation  of  materials  that  the  spaces  between 
larger  pieces  of  the  coarse  aggregate  will  be  occupied  by  smaller  pieces,  and 
the  spaces  between  these  will  in  turn  be  filled  by  sand  until  in  a  perfectly 
proportioned  mixture  there  will  remain  only  such  voids  as  will  be  taken  up 
by  the  cement  paste  when  the  concrete  is  finally  compacted  in  the  place  of 
its  ultimate  use.  The  absolute  elimination  of  voids  is  an  ideal  condition, 
hence  it  is  essential  to  use  every  means  in  our  power  toward  approaching 
the  perfection  suggested.  The  more  nearly  we  approximate  the  theoretical 
possibility,  the  more  successful  we  shall  be  in  actual  practice. 

Object 

Both  strength  and  density  in  finished  concrete  construction  are  depend- 
ent upon  careful  proportioning.  A  very  porous  concrete  may,  under  certain 
conditions  of  manufacture,  be  stronger  than  a  seemingly  dense  concrete 
which  is  lacking  in  cement  or  in  coarse  aggregate.  Hence  we  observe  work 
disintegrate  after  two  or  three  years,  and  upon  examining  a  fracture  find 
that  the  concrete  has  no  large  voids,  but  is  composed  of  fine  sand  with  little 
or  no  coarse  aggregate.  Such  material  may  appear  dense,  but  hardly  de- 
serves to  be  called  concrete. 

On  the  other  hand,  remarkable  instances  of  strength  developed  in  porous 

20 


concrete  may  be  observed  where  the  coarse  aggregate  was  fairly  well  graded 
and  but  little  sand  used.  This  practice  is  not  recommended  because  the 
working  conditions  might  not  be  identical,  and  a  concrete  possessing  a  large 
percentage  of  voids  will  not  be  water-tight.  The  point  is  mentioned  merely 
to  emphasize  the  fact  that  coarse  aggregate  and  cement  give  strength  to 
concrete.  Sand  increases  the  density. 

Impermeability,  or  resistance  to  the  passage  of  water,  is  one  of  the  most 
prominent  characteristics  of  good  concrete  and  is  absolutely  dependent 
upon  the  elimination  of  voids,  which  results  only  from  correct  proportioning 
of  ingredients.  A  porous  concrete  is  never  water-tight.  Quite  a  number  of 
processes  for  waterproofing  have  been  suggested;  some,  like  soap  and  alum, 
or  the  "Sylvester  Process,"  are  public  property,  while  others  are  either 
secret  formulas  or  process  patents.  Some  consist  of  incorporating  com- 
pounds in  the  concrete  at  the  time  of  mixing,  and  others  of  applying  com- 
pounds to  the  exterior  or  interior  of  the  work  after  completion.  If  the  con- 
crete is  properly  proportioned,  there  is  no  reason  for  using  any  integral 
waterproofing  medium. 

In  reinforced  concrete  work  a  satisfactory  bond  between  the  steel  and 
'  concrete  can  be  obtained  only  by  such  careful  proportioning  as  will  insure 
a  concrete  practically  free  from  voids.  This  does  not  mean  merely  slushing 
in  water  enough  to  fill  spaces  between  aggregate  surrounding  rods  or  other 
reinforcement.  Surplus  water  will  disappear  by  evaporation,  leaving  cavi- 
ties adjacent  to  the  reinforcement,  and  when  a  failure  occurs,  rods  will  be 
found  pulled  out  of  porous  concrete,  the  porous  concrete  not  offering  suffi- 
cient bond  to  transfer  the  stress  to  the  steel  reinforcement. 

Methods  of  Proportioning 

One  method  of  proportioning  is  by  measuring  the  amount  of  water 
required  to  fill  the  voids  in  the  coarse  aggregate,  and  using  a  like  proportion 
of  sand,  in  turn  measuring  similarly  the  voids  in  the  sand  to  determine  the 
required  proportion  of  cement.  This  method  of  proportioning  is  inaccurate 
and  cannot  be  recommended  for  general  use. 

Another  method  consists  of  ascertaining  the  specific  gravity  of  the 
material  to  be  used,  then  weighing  a  fixed  volume  of  the  sand,  gravel,  or 
broken  stone  in  the  condition  in  which  it  is  to  be  used,  and  from  the  differ- 
ence between  the  weight  of  like  volumes  of  solid  and  loose  material  determin- 
ing the  percentage  of  voids.  This  method  is  scientifically  correct,  but  will 
seldom  be  used  outside  of  laboratory  practice  on  account  of  the  equipment 
required  to  make  the  computation  accurately. 

There  is  no  doubt  that  density  proportioning  is  the  most  practical  and 
definite  method  yet  evolved.  While  it  is  largely  a  cut  and  try  method, 
and  should  be  checked  by  cylinder  compression  tests,  there  are  fewer  possi- 

21 


bilities  of  error  and  the  results  are  not  dependent  on  the  use  of  delicate 
apparatus.  The  density  test  has  its  value  in  the  determination  of  the 
proper  amount  of  coarse  aggregate  to  use  with  a  given  mortar.  This  does 
not  mean,  of  course,  that  the  determination  of  mortar  density  is  not  of  great 
value  in  obtaining  the  relative  merits  of  two  given  sands,  as  it  might 
develop  in  an  analysis  of  this  kind  that  one  sand  would  work  better  than 
another  in  lean  mixtures  and  poorer  in  rich  mixtures.  Take  a  fixed  weight 
of  dry  coarse  aggregate  and  one-half  the  same  weight  of  dry  sand.  Shake 
them  down  in  a  cylindrical  vessel  and  mark  how  high  the  mixture  fills  the 
vessel;  then  try  another  mixture  of  the  same  total  weight,  but  using  less 
sand  and  more  coarse  aggregate,  or  a  mixture  of  like  weight  using  more  sand 
and  less  coarse  aggregate.  The  relative  proportions  by  weight  which  will 
occupy  the  least  volume  are  the  proportions  containing  the  smallest  possible 
percentage  of  voids.  This  method  is  very  effective  and  requires  neither 
apparatus  nor  technical  skill.  If  conditions  require  proportions  by  volume 
rather  than  weight,  as  is  generally  the  case,  the  experimental  process  will  be 
reversed,  measuring  the  materials  placed  in  the  cylinder  and  trying  different 
compounds  to  ascertain  which  gives  the  greatest  weight  for  the  same  total 
volume. 

In  proportioning  by  volume  a  sack  of  cement  will  be  considered  as  one 
cubic  foot;  in  proportioning  by  weight,  a  sack  of  cement  may  be  accepted  as 
94  pounds  net.  In  determining  the  amount  of  cement  necessary  to  fill 
voids  in  sand,  several  experimental  mixtures  should  be  prepared  in  different 
proportions  and  the  tests  conducted  as  already  described  for  the  inert 
materials.  This  method  of  determining  the  composition  of  mortars  is  also 
highly  recommended  for  determining  a  choice  between  two  or  more  sands 
of  like  composition,  because  the  sand  which  gives  a  mortar  of  least  volume 
for  like  weights  will  always  make  the  densest  concrete. 

Sizing  Materials 

Unless  sand  and  gravel  are  purchased  separately,  it  will  be  necessarj-  to 
separate  them  by  screening  to  arbitrary  sizes  before  proportioning.  If,  for 
instance,  it  is  proposed  to  use  bank  gravel  varying  in  size  from  fine  sand  up 
to  small  boulders,  two  screens  should  be  used,  the  first  rejecting  everything 
exceeding  the  maximum  size  of  aggregate  suitable  for  the  work,  this  varying 
from  %  inch  for  fence  posts  and  block  up  to  2  inches  for  foundations  and 
other  work  of  large  cross-section.  The  general  rule  for  wall  is  that  the 
largest  size  of  aggregate  shall  not  exceed,  in  its  greatest  diameter,  one-half 
the  thickness  of  the  wall.  The  second  screen  should  in  all  cases  be  of  34- 
inch  mesh,  the  particles  retained  upon  it  to  be  regarded  as  coarse  aggregate, 
and  those  passing  it  as  fine  aggregate  or  sand. 

22 


Average  Proportions 

As  many  users  of  concrete  do  not  wish  to  take  the  trouble  to  test  their 
own  materials,  it  is  customary  for  them  to  use  the  proportions  which  have 
been  found  to  produce  satisfactory  results  under  average  conditions. 
These  are  one  part  of  cement,  two  parts  of  sand,  and  four  parts  of  coarse 
aggregate  (expressed  1 : 2 : 4)  for  most  classes  of  construction.  In  the  manu- 
facture of  products  large  enough  to  use  aggregate  exceeding  one  inch  in 
greatest  dimension  the  proportion  of  coarse  aggregate  may  be  increased  ac- 
cordingly. Conversely,  where  a  fine  texture  is  desired  for  ornamental 
purposes,  the  proportion  of  cement  must  be  increased,  reaching  its  maxi- 
mum in  1 :  IJHz  troweled  surfaces.  The  following  table  gives  the  propor- 
tions recommended  for  various  classes  of  work: 

A  1:2:3  mixture  for : 

One-course  concrete  highway,  street,  and  barnyard  pavements. 

One-course  floors  and  walks. 

Roofs. 

Fence-posts  and  for  sills  and  lintels  without  mortar  surface. 

Water-troughs  and  tanks. 

A  1 : 2 : 4  mixture  for : 

Reinforced  concrete  floors,  beams,  and  columns. 
Large  engine  foundations. 
Work  subject  to  vibration. 
Building  walls  above  foundation. 
Silo  walls. 

1 :  2^ :  4  mixture  for : 

Base  of  two-course  street  and  highway  pavements. 
Backing  of  concrete  block  and  similar  cement  products. 

A  1:3:5  mixture  for : 

Supporting  walls  and  foundations. 
Small  engine  foundations. 
Base  of  sidewalks  and  two-course  floors. 
Mass  concrete  footings,  etc. 

MOETAR 

1 :  l*/<2  mixture  for: 
Wearing  course  of  two-course  floors. 

1 : 2  mixture  for : 
Scratch  coat  of  exterior  plaster. 
Facing  blocks  and  similar  cement  products. 
Wearing  course  of  two-course  walks,  street,  and  highway  pavements. 

1:2^  mixture  for: 
Finish  coat  of  exterior  plaster. 
Fence-posts  when  coarse  aggregate  is  not  used. 

23 


1 : 3  mixture  for : 

Concrete  blocks  when  coarse  aggregate  is  not  used. 
Cement  drain  tile  when  coarse  aggregate  is  not  used. 

Amount  of  Water 

The  consistence  will  depend  upon  the  use  for  which  the  concrete  is  in- 
tended and  upon  the  process  of  manufacture  necessarily  associated  there- 
with. 

Three  consistencies  or  mixtures,  determined  by  the  amount  of  water 
used,  are  generally  called  the  dry,  the  quaky,  and  the  wet.  The  dry  mix- 
ture is  of  the  consistence  of  damp  earth,  and  is  used  where  the  concrete 
is  tamped  into  place,  being  principally  useful  in  steel  molds  for  making 
products  requiring  no  reinforcement,  such  as  brick,  block,  and  ornamental 
cases. 

The  quaky  mixture  is  so  named  because  it  is  wet  enough  to  quake  or 
shake  when  tamped.  It  is  used  in  all  molded  products  requiring  reinforce- 
ment, such  as  fence-posts,  lamp-posts,  telegraph  and  telephone  poles,  drain 
tile,  sewer-pipe,  ash-pit  rings,  and  the  like;  also  in  engine  foundations  and 
the  footings  of  buildings. 

The  wet  mixture  contains  sufficient  water  to  permit  of  its  flowing  from 
a  shovel  or  wheelbarrow,  but  not  enough  to  cause  a  separation  of  the  par- 
ticles. It  is  used  in  building  reinforced  concrete  structures,  such  as  silos, 
barns,  dwellings,  and  other  buildings  where  the  concrete  is  allowed  to 
remain  undisturbed  in  the  forms  for  several  weeks.  The  scum  (or  laitance) 
should  be  scraped  from  the  surface  of  green  concrete  and  the  surface  thor- 
oughly scrubbed  and  moistened  before  placing  additional  concrete. 

There  is  a  pronounced  tendency  at  the  present  time  to  use  too  much 
water.  This  results  in  concrete  which  is  porous  and  of  very  low  initial 
strength.  There  are  very  few  instances  in  actual  construction  work  where 
a  plastic  wet  mix  will  not  be  satisfactory  and  a  word  of  warning  should  be 
sounded  against  the  use  of  very  wet,  sloppy  mixtures. 

II.  Mixing 
Fundamental  Principle 

The  importance  of  thoroughly  and  carefully  mixing  the  ingredients  used 
in  the  manufacture  of  concrete  is  secondary  only  to  the  proportioning, 
because  the  mixing  cannot  be  done  until  after  the  proportioning  has  been 
accomplished.  It  is  secondary  in  time,  but  equal  in  importance. 

As  stated  earlier  in  this  lesson,  an  essential  feature  of  concrete  construc- 
tion is  the  coating  of  every  grain  of  sand  with  a  film  of  neat  cement,  and  the 
coating  of  every  piece  of  coarse  aggregate  with  sand-cement  mortar.  This 
statement  may  be  emphasized  by  stating  that  it  is  the  fundamental  prin- 

24 


ciple  of  all  concrete  construction;  an  earnest  effort  to  accomplish  this  result 
will  insure  success. 

In  machine  mixing  experiments  show  that  for  periods  up  to  two  minutes 
the  strength  of  concrete  made  from  the  same  materials  and  with  the  same 
percentage  of  water  is  proportional  to  the  time  it  is  kept  in  the  revolving 
mixer. 

Assuming  that  proper  proportions  have  been  determined,  the  result  so 
carefully  sought  can  be  attained  only  by  thorough  and  intelligent  mixing. 

Shovel  Mixing 

Let  us  first  consider  the  rather  difficult  problem  of  securing  satisfactory 
results  where  the  volume  of  work  does  not  warrant  the  installation  of  a 
mixing  machine. 

The  first  requirement  will  be  a  water-tight  platform  large  enough  for  two 
men  to  shovel  conveniently  from  either  end  as  large  a  batch  of  concrete  as 
can  be  used  within  thirty  minutes  after  water  has  been  added  to  it. 

If,  on  account  of  meal-time  or  any  emergency,  a  portion  of  a  batch  lies 
until  the  cement  has  become  partially  hardened,  throw  it  away  rather 
than  jeopardize  the  work. 

As  proportioning  is  usually  done  by  volume,  one  cubic  foot  is  a  conve- 
nient unit,  as  it  allows  full  sacks  of  cement  to  be  used.  The  required  amount 
of  sand  should  first  be  spread  upon  the  mixing  platform,  after  which  the 
cement  should  be  spread  in  a  layer  on  the  sand.  Two  men,  using  square 
pointed  shovels,  will  then  turn  the  sand  and  cement  over  two  or  more  times 
until  the  streaks  of  brown  and  gray  have  merged  into  a  uniform  color 
throughout  the  mass.  The  coarse  aggregate  is  then  added  and  the  mix- 
ing continued,  water  being  added  during  the  first  turning  after  adding 
coarse  aggregate.  Water  should  be  added  gently,  as  from  a  hose  nozle  or 
the  spout  of  a  watering-pot,  in  order  to  prevent  washing  out  the  cement. 
Turning  should  continue  until  the  mortar  is  of  uniform  consistence  through- 
out, which  will  usually  require  at  least  three  turnings  after  adding  water. 

Mixing  in  the  above  manner  will  give  satisfactory  results,  but  the  labor 
involved  is  considerable,  and  on  this  account  it  is  too  common  for  those 
attempting  it  to  slight  the  work  and  use  the  concrete  in  an  imperfectly 
mixed  condition. 

Machine  Mixing 

Mixers  have  been  brought  to  a  high  state  of  efficiency,  and  to-day  there 
are  many  on  the  market  designed  to  produce  the  best  results  at  minimum 
cost  of  labor  and  power.  While  it  is  beyond  the  scope  of  this  lesson  to  dis- 
cuss mixers,  we  may,  in  passing,  mention  one  or  two  of  the  principles  which 
will  assist  the  concrete  manufacturer  in  making  a  selection  suited  to  his 

25 


needs.  The  batch-mixers,  whether  cubes,  cylinders,  or  truncated  cones, 
allow  the  material  to  be  introduced  in  any  order  desired,  provided  only  that 
each  separate  batch  contains  the  proper  relative  proportions  of  ingredients. 
After  the  batch  has  been  placed  in  the  mixer,  it  is  revolved  for  a  specified 
time,  or  a  definite  number  of  revolutions,  until  either  by  the  shape  of  the 
drum  itself  or  by  means  of  deflectors  therein  the  cement,  sand,  and  coarse 
aggregate  have  been  thoroughly  mixed.  Most  batch-mixers  are  equipped 
with  a  small  tank  from  which  a  pipe  leads  into  the  mixer,  and  when  the 
materials  have  been  sufficiently  mixed  in  a  dry  state,  water  is  sprayed  on 
them  while  the  revolutions  of  the  mixer  continue. 

The  continuous  mixer  consists  mainly  of  a  number  of  hoppers  for  the 
several  materials,  placed  over  one  end  of  a  semi-circular  trough  containing 
blades  or  shovels  fixed  to  a  rotating  shaft.  The  motive  power  is  generally 
supplied  by  a  gasolene  engine  or  an  electric  motor.  The  dry  materials  are 
fed  automatically  from  the  hoppers  into  the  trough,  mixed,  and  carried 
along  by  the  blades  to  the  discharge  end,  water  being  added;  meanwhile 
the  concrete  is  discharged  continuously. 

The  batch  type  of  mixer  is  considered  by  the  majority  of  engineers  to 
give  the  best  results  because  the  measuring  of  the  materials  can  be  posi- 
tively regulated,  whereas  with  the  continuous  mixer  variations  in  the  amount 
of  moisture  in  the  sand  or  flumness  of  the  cement  will  cause  a  variation  in 
the  relative  proportions  of  these  materials  in  the  mixture.  On  this  account 
engineers  favor  the  batch-mixer. 

Lists  of  manufacturers  of  mixers  will  be  found  in  the  columns  of  current 
concrete  periodicals. 

III.  Placing 
Final  Problem 

But  when  all  is  said  and  done :  when  we  have  selected  the  best  materials, 
have  ascertained  the  proper  proportions  of  each  and  the  correct  amount  of 
water  for  the  consistence  required  to  serve  our  particular  purpose;  when  by 
shovel  or  machine  we  have  combined  the  different  materials  required  to 
make  concrete,  we  have  produced  a  mass  of  material  which  must  be  care- 
fully deposited,  compacted,  and  made  to  take  some  one  of  the  thousand 
and  one  shapes  which  concrete  assumes. 

This,  then,  is  our  problem,  the  placing  of  the  concrete,  and  we  shall  find 
three  distinct  methods  of  accomplishing  this  result: 

Pressure  and  Tamping 

Whenever  a  dry  mixture  is  used  in  steel  molds  to  produce  such  unrein- 
forced  products  as  ornamental  vases,  block  or  brick,  concrete  is  placed  by 

26 


pressing  or  tamping.  If  pressure  is  applied,  it  will  ordinarily  be  by  means 
of  a  press  simplifying  the  process  and  making  it  necessary  only  to  see  that 
the  molds  are  adequately  and  evenly  filled  in  order  that  the  product  may  be 
uniform  in  density.  If,  however,  tamping  is  the  method  employed,  con- 
siderable supervision  will  be  found  necessary,  as  the  quality  of  the  product 
may  vary  considerably  unless  the  tamping  is  uniformly  performed.  It  is 
particularly  necessary  that  the  mold  be  tamped  while  filling,  not  filled  and 
tamped  afterward.  The  latter  method  will  not  only  fail  to  fill  the  lower 
corners,  but  will  make  one-half  of  the  molded  product  much  denser  than  the 
other.  If  tamping  is  well  done  by  one  man  (or  two,  if  a  large  mold)  while 
the  mold  is  being  filled  by  another,  there  is  no  reason  why  the  product 
should  not  be  perfectly  satisfactory  and  as  uniform  as  though  made  under 
mechanical  or  hydraulic  pressure.  To  secure  more  uniform  density  and 
effect  a  saving  of  labor,  power  tampers  are  used,  the  multiple  tampers  being 
especially  serviceable  in  making  block  and  brick. 

Agitation 

Neither  tamping  nor  pressure  will  be  of  service  in  the  case  of  those 
products  requiring  the  introduction  of  reinforcement,  such  as  tile,  pipe, 
poles,  and  posts.  In  the  manufacture  of  these  and  similar  products  the 
steel,  in  whatever  form  required  for  reinforcing,  is  introduced  at  the  proper 
place  in  the  mold  while  it  is  being  filled,  with  a  quaky  mixture  of  concrete 
which  is. compacted,  forced  into  corners  and  around  or  through  the  reinforce- 
ment, by  vigorously  stirring  the  mixture  and  jarring  the  mold. 

Depositing  Wet  Concrete 

Placing  concrete  for  reinforced  concrete  structures,  including  silos  and 
all  sorts  of  buildings,  involves  work  on  a  scale  warranting  the  installation  of 
special  apparatus  to  save  both  time  and  labor  in  transporting  the  concrete 
from  the  mixer  to  the  place  of  use.  Elevators,  dump-cars,  and  chutes  are 
ordinarily  used  in  the  construction  of  reinforced  concrete  buildings.  In 
constructing  silos  it  is  economical  to  provide  a  center  hoisting  device  with 
derrick  and  an  automatic  dumping  bucket. 

The  concrete  is  poured  into  forms  in  which  reinforcement  has  previously 
been  placed.  It  is  then  necessary  to  spade  it  back  from  the  forms  in  order 
to  prevent  large  pieces  of  aggregate  from  retaining  surface  positions  when 
the  forms  are  removed.  The  larger  pieces  of  aggregate  should,  as  far  as 
possible,  be  forced  away  both  from  the  reinforcement  and  the  forms,  so  that 
they  may  occupy  an  intermediate  position.  Though  the  subject  of  forms  is 
treated  in  another  lesson,  a  word  of  caution  relative  to  their  removal  may 
not  be  amiss  at  this  time.  While  no  definite  rule  can  be  given  to  fit  all  local 
conditions  and  variations  of  structure,  humidity,  and  temperature,  good 

27 


judgment  will  suggest  that  too  early  removal  involves  clanger,  while  reason- 
able delay  in  removing  forms  is  a  wise  precaution,  insuring  safety. 

REFERENCES 

"  Concrete,  Plain  and  Reinforced,"  by  Taylor  and  Thompson.     Published  by  John  Wiley 

and  Sons,  New  York  City. 
"Concrete  Costs,"  by  Taylor  and  Thompson.     Published  by  John  Wiley  and  Sons, 

New  York  City. 
"Reinforced  Concrete  Construction,"  by  Buell  and  Hill.     Published  by  McGraw-Hill 

Book  Co.,  New  York  City. 


IV.  Forms 

Introductory 

The  plasticity  of  concrete,  and  the  readiness  with  which  the  material  can 
be  adapted  to  all  shapes  and  sizes  of  construction,  have  from  the  beginning 
of  the  more  extensive  use  of  concrete  made  the  production  of  molds  of 
desired  form  a  very  important  consideration  in  all  concrete  construction 
work.  While  iron  and  steel  molds  have  been  used  for  small  members,  such 
as  block  and  brick  and  ornamental  pieces,  in  which  the  same  design  and 
size  can  be  indefinitely  repeated,  larger  concrete  construction  requires  indi- 
vidual design,  determined  by  local  conditions  and  particular  needs.  The 
ease  with  which  concrete  may  be  adapted  to  such  peculiar  requirements  of 
individual  use  is  one  of  the  chief  merits  of  the  material.  Consequently, 
means  must  be  provided  for  constructing,  at  or  near  the  place  where  the 
concrete  is  to  be  used  and  from  materials  easily  procured,  molds  which  may 
be  made  to  fit  the  circumstances  of  each  individual  case.  Molds  of  this 
diversified  character  are  commonly  called  forms. 

Classification 

Forms  may  be  roughly  classified  as  follows: 

1.  Rectangular  forms  wholly  of  lumber. 

2.  Rectangular  forms  using  metal  fastening  devices. 

3.  Rectangular  metal  forms. 

4.  Circular  forms  of  wood  and  sheet  metal. 

5.  Circular  forms  wholly  metal. 

6.  Miscellaneous. 

Lumber  Forms 

Contrary  to  the  usual  practice  in  building  construction,  green  lumber 
will  keep  its  shape  in  all  rectangular  forms  better  than  lumber  that  is  thor- 
oughly dry.  If  dry  lumber  is  used,  it  should  be  thoroughly  wet  before  the 
concrete  is  placed.  The  use  of  oil  or  grease  free  from  animal  oils  or  fats  on 

28 


the  inside  of  forms  is  recommended,  as  it  prevents  absorption  of  water  from 
the  concrete  by  the  forms  and  makes  their  removal  easier.  Where  any  fine 
ornamentation  is  used,  the  molding  or  other  device  introduced  to  vary  the 
surface  should  be  painted  with  equal  parts  of  boiled  linseed  oil  and  kerosene, 
it  is,  however,  essential  that  forms  should  be  thoroughly  cleaned  each  time 
they  are  used,  and  that  no  dry  concrete  be  left  sticking  to  the  face  of  the 
forms.  Forms  may  be  built  from  stock  length  lumber,  requiring  very  little 
sawing  and  permitting  of  the  lumber  being  used  later  for  other  purposes. 
White  pine  is  considered  the  best  lumber  for  forms,  although  spruce,  fir,  and 
Norway  pine  are  often  used.  The  face  of  forms  should  be  free  from  loose 
knots,  slivers,  or  other  irregularities,  as  concrete  will  reproduce  them  all  with 
great  faithfulness.  Matched  lumber  may  be  used  to  afford  a  smooth  finish, 
and  very  satisfactory  results  can  be  obtained  by  proper  care  in  the  con- 
struction of  forms. 

Rectangular  Forms 

In  the  construction  of  rectangular  forms  the  first  type  of  construction 
presenting  itself  for  consideration  is  foundation  work.  Where  the  excava- 
tion is  made  simply  for  a  foundation  without  cellar  or  basement,  the  soil  will 
often  be  firm  enough  so  that  the  trench,  if  carefully  excavated,  may  be  used 
as  a  form  below  ground  line.  In  this  case  the  edges  must  be  protected  to 
keep  the  dirt  out  of  the  concrete.  In  carrying  the  foundation  from  the 
ground  line  to  the  level  of  the  first  floor,  forms  must  be  constructed  resting 
upon  a  bridge  and  extending  slightly  below  the  ground  line.  These  forms 
may  be  constructed  in  sections  or  built  in  place. 

If  the  inner  and  outer  parts  of  the  form  are  built  separately,  they  must, 
when  put  into  position,  be  leveled  and  plumbed  carefully.  Whether  built  in 
sections  or  built  in  place,  forms  must  be  braced  thoroughly  and  tied  to- 
gether, as  the  essential  duty  of  any  form  is  rigidly  to  maintain  its  integrity 
until  the  concrete  has  hardened. 

Forms  for  foundation  piers  and  for  the  foundation  of  all  kinds  of  ma- 
chinery are  constructed  in  substantially  the  same  manner  as  for  regular 
building  foundations.  The  construction  of  machinery  foundations  is  essen- 
tially a  problem  of  securing  the  necessary  mass  and  weight,  consequently 
the  greater  part  of  the  foundation  will  be  under  ground,  and  all  that  is 
required  above  ground  is  an  open  box  of  sufficient  strength  to  maintain  the 
concrete  in  the  desired  form  while  hardening. 

Where  the  excavation  for  cellar  is  made  by  team  and  scraper,  the  sides 
will  not  be  perpendicular,  and  the  excavation  will  usually  be  somewhat 
larger  than  the  dimensions  of  the  cellar-wall.  Consequently  it  is  necessary 
to  use  both  inner  and  outer  forms.  Each  form  is  braced  by  uprights  spaced 
close  enough  to  prevent  any  spreading  or  bulging  of  the  sheeting  when  sub- 

29 


jected  to  the  outward  pressure  of  the  fresh  concrete.  The  inner  form  should 
be  securely  braced  in  a  perpendicular  position  by  lumber  braces  from  the 
floor  of  the  excavation.  The  outer  form  should  be  fastened  to  the  inner 
form  by  wires  running  through  both  near  the  bottom,  and  at  the  same  place 
the  forms  should  be  separated  by  spacing  blocks  of  the  width  determined 
upon  for  the  cellar  wall.  The  outer  form  should,  like  the  inner,  be  perpen- 
dicular unless  a  slight  batter  is  desired,  in  which  case  the  spacing  blocks 
should  be  lengthened  to  spread  the  bottom  of  the  forms  apart  and  increase 
the  thickness  of  the  wall  at  the  bottom  without  interfering  with  the  estab- 
lished thickness  of  the  wall  at  the  top.  The  wires  connecting  the  two  forms 
should  be  drawn  tight  by  twisting  with  a  large  nail  or  rod  until  the  forms  are 
drawn  firmly  against  the  spacing  blocks.  The  top  of  the  uprights  should  be 
joined  by  cleats.  The  method  just  described  produces  a  very  rigid  form. 

When  an  outside  cellar  or  basement  entrance  is  desired,  the  forms  for 
same  should  be  constructed  simultaneously  with  the  cellar  wall  forms. 
When  in  position,  these  forms  will  rest  upon  the  floor  of  the  excavation  made 
for  the  steps.  If  the  excavation  for  the  entrance  is  carefully  made,  only  the 
inside  form  will  be  required  until  ground  line  is  reached.  As  the  walls  will 
project  above  the  ground  where  they  join  the  building  and  slope  from  that 
point  to  the  opposite  end  of  the  entrance,  an  outside  form  will  be  required 
above  the  ground  line.  By  properly  bracing  the  form,  one  side  wall  may  be 
made,  and  after  it  has  hardened  the  form  reversed  and  used  for  the  other 
side.  After  both  side  walls  have  been  made,  forms  for  the  steps  giving 
desired  height  of  riser  and  width  of  tread  may  then  be  securely  braced 
between  the  side  walls. 

In  the  construction  of  double  walls,  such  as  in  ice-houses,  the  intervening 
air-space  is  not  usually  wide  enough  to  accommodate  two  sets  of  forms. 
Therefore  the  hollow  wall  is  usually  constructed  by  placing  in  the  forms 
cores  which  are  later  withdrawn. 

Forms  for  walks  and  floors  should  consist  of  2-inch  lumber,  in  width 
equal  to  the  desired  thickness  of  the  walk  or  floor,  staked  in  the  earth  to 
form  slabs  of  the  desired  size.  The  concrete  is  mixed  wetter  than  for  two- 
course  work,  and  where  the  walk  or  floor  is  laid  in  one  course,  slabs  should 
be  laid  alternately,  allowing  cross  forms  to  remain  in  place  until  ready  to  fill 
intermediate  slabs.  This  method  is  also  used  extensively  in  two-course 
work,  although  many  prefer  to  work  consecutively,  moving  the  cross-piece 
each  time  a  slab  is  completed.  If  laid  continuously,  care  must  be  exercised 
to  preserve  the  vertical  joints  through  the  entire  walk.  Horse  blocks  or 
carriage  steps  may  be  constructed  where  the  walk  joins  the  driveway  by  the 
use  of  simple  box  forms. 

The  modern  farmer  is  making  use  of  concrete  for  the  construction  of 
various  types  of  tanks,  such  as  the  stock  watering  tank,  the  hog  feeding 

30 


trough,  the  dipping  vat,  and  the  hog  wallow,  all  of  which  may  be  constructed 
by  the  use  of  rectangular  lumber  forms. 

The  general  method  of  constructing  rectangular  tanks  above  ground 
consists  in  erecting  an  outer  form,  usually  of  2-inch  lumber,  in  which  the 
concrete  floor  of  the  tank  is  placed,  and  the  surface  finished  as  desired,  after 
which  the  bottomless  inner  form,  which  must  be  previously  prepared  and 
ready  for  immediate  use  before  the  previously  placed  concrete  has  hardened, 
is  quickly  inserted  and  securely  fastened  in  place  by  cleats  joining  the  up- 
rights of  the  outer  and  inner  forms.  The  method  of  constructing  rectangu- 
lar tanks  underground  differs  only  in  that  the  earth  usually  forms  the  outer 
form  and  a  wood  form  is  required  for  the  roof.  In  constructing  septic  tanks 
provision  must  be  made  for  the  several  partitions  and  compartments  neces- 
sary to  secure  decomposition  of  the  sewage  and  disposal  of  the  effluent. 

Two  methods  are  used  with  equal  satisfaction  in  manufacturing  small 
troughs,  which  need  not  necessarily  be  built  in  place.  One  is  to  use  a  box 
mold  and  finish  the  interior  with  a  straight  batter  or  a  concave  surface  by 
striking  it  out  with  a  templet.  The  other  method  is  to  use  a  core  of  firm 
clay  or  wood  made  in  shape  to  correspond  with  the  inside  of  the  trough. 
A  bottomless  box  is  placed  over  the  inverted  core,  and  by  filling  the  box 
with  concrete  and  striking  it  off  level,  the  trough  is  manufactured  upside 
down. 

The  simplest  deviation  from  home-made  molds  is  to  purchase  clamps  for 
holding  forms  in  place,  thus  doing  away  with  nailing  them  to  the  uprights. 
There  are  several  systems  of  clamps  on  the  market,  some  of  which  are  very 
ingenious,  and  all  of  which  are  designed  with  two  purposes  in  view,  the  first 
being  to  facilitate  the  erection  and  removal  of  forms,  and  the  second  being 
to  save  loss  of  lumber  from  repeated  nailing  and  tearing  down. 

A  still  wider  departure  from  the  home-made  forms  brings  us  to  those 
constructed  wholly  of  metal,  which  provide  a  rapid  and  economical  method 
of  concrete  construction  where  a  large  amount  of  work  is  to  be  done  along 
uniform  lines.  Only  continued  repetition,  however,  will  justify  the  pur- 
chase of  metal  forms.  Where  the  opportunity  occurs  to  rent  metal  forms 
for  any  work  of  considerable  importance,  a  saving  may  be  effected  and  the 
quality  of  the  work  somewhat  improved  on  account  of  greater  surface 
uniformity  secured  by  use  of  the  metal  forms. 

Circular  Forms 

Circular  forms  are  extensively  used  in  the  construction  of  tanks  because 
a  round  tank  is  more  economical  to  build  and  will  resist  frost  action  better 
than  a  tank  of  any  other  shape.  The  construction  of  a  circular  form  pre- 
sents greater  difficulty  than  does  that  of  a  rectangular  form,  and  it  is  usually 
better  for  several  of  those  who  desire  to  construct  tanks  to  determine  upon 

31 


a  standard  size  and  join  in  the  use  of  a  set  of  forms,  or,  if  this  cannot  be 
done,  a  set  of  forms  may  be  rented  if  but  a  single  tank  is  to  be  made.  For  a 
10-foot  circular  tank,  2  feet  6  inches  in  depth,  the  forms  usually  cost  about 
$50,  while  the  cost  of  the  tank  itself,  exclusive  of  sand  and  gravel,  is  only 
$30.  Forms  for  circular  tanks  consist  of  an  inner  and  an  outer  wooden 
frame  covered  with  sheet  iron.  Silo  forms  may  be  used  for  the  outer  forms 
of  large  tanks.  The  height  of  the  inner  form  is  equal  to  the  inside  depth  of 
the  tank,  and  the  height  of  the  outer  form  is  equal  to  the  sum  of  the  inside 
depth  and  the  floor  thickness  of  the  tank.  After  the  inner  and  outer  circles 
of  the  form  have  been  laid  out,  segments  are  cut  from  1-inch  lumber  and  a 
wooden  frame  is  built  up,  fence  fashion.  No.  22  gage  galvanized  iron  is 
then  attached  by  screws  or  nails.  The  inner  form  should  slope  toward 
the  outer  one,  to  give  proper  batter  to  the  inside  of  the  tank,  and  prevent 
bursting  in  case  of  freezing. 

The  selection  of  silo  forms  presents  to  the  modern  farmer  one  of  the 
most  important  problems  in  connection  with  the  use  of  concrete.  What  are 
known  as  home-made  silo  forms  are  usually  constructed  in  3-foot  sections, 
but  it  is  hardly  desirable  to  construct  a  set  of  forms  for  the  express  purpose 
of  building  one  silo.  It  is  far  better  for  farmers  to  unite  in  the  matter,  as  a 
set  of  forms  may  be  used  for  constructing  a  large  number  of  silos.  How- 
ever, if  one  must  build  his  own  forms,  a  most  ingenious  model  is  that  of  Mr. 
David  Imrie,  Roberts,  Wisconsin,  who  has  introduced  his  form  to  hundreds 
of  farmers  in  connection  with  the  work  of  the  Wisconsin  Farmers'  Institute. 
The  inner  form  consists  essentially  of  hooped  sheet  metal  securely  clamped 
and  braced.  No.  28  gage  galvanized  sheet  iron  is  used,  and  the  form  is 
assembled  in  eight  segments  which  are  bolted  together.  The  outer  form  is 
made  of  18  or  20  gage  galvanized  sheet  metal  3  feet  in  width,  in  two  or  more 
pieces,  joined  by  heavy  band  iron  riveted  to  the  ends  of  each  piece,  which  is 
turned  at  right  angles  and  drilled  to  receive  the  bolts  drawing  adjoining 
sections  together.  Forms  of  this  type  have  been  built  at  a  cost  varying 
from  $25  to  $50. 

Practically  all  silos  now  built  are  roofed.  The  construction  of  the  roof 
form  is  a  simple  matter,  requiring  only  a  box  for  the  cornice  and  2  inch  by 
6  inch  rafters  radiating  from  the  apex  to  the  roof  edge,  on  which  1  inch  by 
6  inch  sheeting  is  laid  to  receive  the  concrete. 

Many  commercial  systems  of  silo  construction  are  now  upon  the  market. 
Fortunately,  most  of  them  are  meritorious  and  will  result  in  more  satis- 
factory work  than  can  be  obtained  from  home-made  molds  inasmuch  as 
they  effect  many  economies  in  the  methods  of  handling  materials  and 
assembling  the  forms.  The  various  commercial  silo  systems  are  operated 
under  different  methods.  The  forms  are  constructed  wholly  of  metal,  and 
some  companies  sell  them  outright  to  an  association  of  farmers  who  desire 

32 


to  construct  silos;  some  companies  rent  their  forms  for  the  construction  of 
a  single  silo;  some  companies  construct  a  silo  for  the  farmer,  acting  in  the 
capacity  of  contractors  and  guaranteeing  their  work  in  every  way. 

Miscellaneous  Forms 

The  miscellaneous  uses  of  concrete  about  the  barn,  barnyard,  and  farm 
in  general  are  innumerable.  The  preparation  of  forms  for  the  many  uses  to 
which  concrete  may  be  put  affords  pleasant  exercise  for  the  ingenuity  of  any 
one  familiar  with  the  uses  of  concrete.  A  few  of  the  possibilities  of  smaller 
construction  are  merely  suggested:  concrete  stalls,  mangers,  hens'  nests, 
hotbeds,  pits  for  wagon  scales,  curbing  for  old  wells,  pump  pits,  and  waste- 
water  receptacles.  The  form  for  the  last  mentioned  consists  of  earth  exca- 
vation for  the  outer  form  and  an  empty  half  barrel  for  the  inner  form,  which 
indicates  how  simple  concrete  construction  may  be  made. 

The  removal  of  the  form  is  a  matter  requiring  very  careful  consideration. 
A  great  deal  of  work  has  been  injured  and  not  a  little  has  failed  because  of 
undue  haste  in  removing  forms.  Two  or  three  days'  additional  time  al- 
lowed to  new  concrete  before  removing  the  forms  often  marks  the  differ- 
ence between  defective  and  thoroughly  satisfactory  work. 

REFERENCES 

"Reinforced  Concrete  Construction,  "by  George  A.  Hool.  Published  by  McGraw- 
Hill  Book  Company,  New  York  City. 

"Concrete  Costs,"  by  Taylor  and  Thompson.  Published  by  John  Wiley  and  Sons, 
New  York  City. 

"Reinforced  Concrete  Construction,"  by  Buell  and  Hill.  Published  by  McGraw-Hill 
Book  Co.,  New  York  City. 


V.    Concrete  Foundations  and  Walls 
I.  Foundations 

Advantages 

Concrete  is  especially  adapted  for  use  in  building  foundations  because  of 
the  following  characteristic  qualities : 

1.  Compressive  strength. 

2.  Durability. 

3.  Moderate  cost. 

4.  Ease  of  construction. 

5.  Adaptability  to  irregular  excavations. 

6.  Capacity  for  reinforcement. 

7.  Can  be  placed  under  water. 

Plain,  or  unreinforced,  concrete  shows  its  greatest  strength  under  direct 
3  33 


compression.  Carrying  capacity  is  the  quality  chiefly  sought  in  the  se- 
lection of  material  for  the  foundation  of  any  building.  Moreover,  concrete 
lasts  forever  without  repairs,  and  permanence  is  a  consideration  scarcely 
secondary  to  strength  in  determining  a  choice  of  foundation  material.  The 
cost  of  a  well-built  concrete  foundation  is  considerably  less  than  that  of  one 
constructed  of  any  other  building  material  of  equal  strength  and  durability. 
Under  average  conditions  the  time  required  for  building  a  concrete  founda- 
tion is  shorter  than  that  required  for  one  of  brick  or  stone.  Concrete  is  the 
only  foundation  material  which  readily  adapts  itself  to  slopes,  change  of 
grade,  or  other  irregularities  in  the  subgrade  on  which  the  foundation  is  laid. 
Wherever  conditions  require  a  foundation  of  restricted  area  on  a  side  hill, 
exposing  a  portion  of  the  foundation  wall  to  danger  of  accidental  injury,  or 
vibration  of  engines  and  other  machinery  must  be  withstood — in  any  of 
these  cases  concrete  demonstrates  its  adaptability  by  permitting  the  intro- 
duction of  sufficient  reinforcement  satisfactorily  to  perform  the  duty  de- 
manded. 

Owing  to  the  fact  that  Portland  cement  has  the  property  of  hardening 
under  water  it  is  now  almost  universally  used  for  construction  work  carried 
on  below  water.  Care  should  always  be  exercised  not  to  deposit  it  in 
running  water,  inasmuch  as  the  water  will  carry  away  a  portion  of  the 
cement  and  thus  decrease  the  strength  of  the  concrete. 

Consequently,  concrete  is  supplanting  all  other  materials  for  building 
foundations  of  every  character,  irrespective  of  the  character  of  the  super- 
structure. Some  of  the  principles  which  must  be  observed  to  secure  the 
best  results  will  be  here  outlined : 

Materials 

The  proportioning,  mixing,  and  placing  of  concrete  has  been  thoroughly 
discussed  in  another  lesson,  and  the  practices  therein  recommended  should 
be  rigidly  observed.  Further,  it  is  often  possible  in  foundation  work  to 
increase  the  size  of  the  largest  aggregate  up  to  2  inches  or  even  2^  inches. 
Wherever  large  sizes  of  clean,  hard,  durable  gravel  or  broken  stone  can  be 
used,  additional  strength  is  secured;  for  this  purpose  field  stones  may  be 
employed  advantageously. 

Excavation 

In  preparing  for  the  erection  of  any  rectangular  structure  a  base  line 
should  first  be  determined  upon,  and  from  the  base  line  the  several  corners 
should  be  ascertained  by  accurate  measurement  at  right  angles  or  at  such 
other  angles  as  may  be  desired  in  structures  of  irregular  shape.  The  corners 
should  be  staked  and  definitely  fixed  by  a  tack  driven  in  the  top  of  each 
stake.  All  measurements  and  angles  should  then  be  checked  back  to  the 

34 


base  line.  Several  feet  outside  of  the  line  of  stakes  other  stakes  should  be 
set  to  overreach  the  corners,  or  a  frame  may  be  built  10  inches  above  ground, 
called  a  batten  board,  from  which  lines  are  then  run  to  pass  exactly  over  the 
tacks  set  in  the  stakes.  These  lines  show  the  outside  of  the  proposed  exca- 
vation, and  by  measuring  the  width  of  the  foundation  and  running  parallel 
lines  that  far  inside  of  the  first  lines,  the  lay-out  is  ready  for  excavation. 

The  depth  and  width  of  excavation  depend  upon  the  height  and  char- 
acter of  the  building  to  be  erected,  but  should  always  go  to  solid  earth,  and 
should  at  least  be  lower  than  frost  line.  If  the  ground  is  filled  with  surface 
water  at  certain  seasons  of  the  year,  drainage  should  be  provided  from  the 
bottom  of  the  foundation  trench  to  a  natural  outlet. 

Footings 

As  a  convenience  in  setting  forms,  footings  are  sometimes  provided 
where  ground  is  firm.  Wherever  a  foundation  is  to  be  constructed  on  filled 
ground,  which  cannot,  by  rolling  and  tamping,  be  made  solid  enough  to 
guarantee  the  permanent  carrying  without  settlement  of  the  superimposed 
load,  the  weight  must  be  distributed  by  a  layer  of  concrete  wider  than  the 
foundation  itself.  This  is  known  as  a  footing.  It  may  be  twice  as  wide  as 
the  foundation,  but  must  be  thick  enough  to  prevent  shearing  or  cracking, 
and  may  have  either  sloping  or  stepped  sides.  In  extreme  cases  of  very 
soft  earth  requiring  excessively  wide  footings  cross-bars  or  reinforcing  rods 
are  introduced  in  the  footings  to  distribute  the  foundation  load  without 
injury  to  the  concrete  slab. 

Kidder  ("Architects'  and  Builders'  Pocket-book")  gives  bearing  power 
of  soils  as  follows: 

BEARING  POWER  IN  TONS 

PER  SQUARE  FOOT 

Miii.  Max. 

Rock — the  hardest — in  thick  layers  in  native  bed 200 

Rock  equal  to  best  ashlar  masonry 25  30 

Rock  equal  to  best  brick  masonry 15  20 

Rock  equal  to  poor  brick  masonry 5  10 

Clay  on  thick  beds,  always  dry 4  6 

Clay  on  thick  beds,  moderately  dry 2  4 

Clay,  soft 1  2 

Gravel  and  coarse  sand,  well  cemented 8  10 

Sand,  compact  and  well  cemented 4  6 

Sand,  clean,  dry 2  4 

Quicksand,  alluvial  soils,  etc 0.5  1 

Concrete  for  footings  should  be  mixed  in  the  proportions  of  1  sack  of 
Portland  cement,  3  cubic  feet  of  clean,  coarse  sand,  and  5  cubic  feet  of 
gravel  or  broken  stone,  varying  in  size  from  }/±  inch  up  to  2  inches;  if  rein- 
forced, the  proportions  should  be  1:2:4.  Enough  water  should  be  used  to 
form  a  quaky  mixture,  but  not  enough  to  cause  the  cement  and  aggregate 
to  separate  in  placing.  Concrete  foundations  and  footings  may  be  keyed 

35 


by  partially  embedding  in  the  footing  vertical  rods  or  horizontal  I-beams; 
in  light  structures  a  similar  effect  may  be  produced  by  casting  on  the  foot- 
ing a  central  longitudinal  projection  which  will  form  a  tongue-and-groove 
joint  with  the  foundation.  If  the  placing  of  the  foundation  is  delayed 
until  the  footing  has  hardened,  the  latter  should  be  cleaned,  roughened, 
and  wetted,  and  then  grouted  with  a  mixture  in  the  proportion  of  1  sack 
of  Portland  cement  to  1  cubic  foot  of  sand,  mixed  to  the  consistence  of 
thick  cream. 

Simple  Foundations 

Where  there  is  to  be  no  cellar  or  basement  under  a  building  and  the 
nature  of  the  ground  is  such  that  the  excavation  can  be  made  for  the  exact 
width  of  the  foundation,  forms  below  ground  line  are  unnecessary  provided 
the  earth  is  firm  enough  to  prevent  " caving  in"  of  the  sides.  It  is,  how- 
ever, necessary  to  protect  the  edges  and  sides,  especially  on  the  side  opposite 
that  from  which  the  concrete  is  poured,  by  burlap  aprons  made  by  tacking 
a  piece  of  burlap  on  a  piece  of  lumber  2  by  4  inches,  long  enough  to  rest  on 
cross-pieces  bridging  the  excavation.  When  the  ground  line  is  almost 
reached,  forms  previously  constructed  of  1-inch  boards  on  2-inch  by  4- 
inch  studding  must  be  placed  to  receive  the  concrete  from  ground  line  to 
the  top  of  the  foundation  wall.  The  forms  must  not  rest  on  the  concrete 
already  placed,  but  upon  a  bridge  which  will  allow  them  to  drop  slightly 
below  the  ground  line.  No  appreciable  time  should  elapse  between  plac- 
ing the  concrete  below  and  above  ground,  as  an  interval  of  more  than  thirty 
minutes  will  produce  a  line  of  cleavage,  seriously  weakening  the  wall  and 
lessening  its  water-tightness. 

Piers  and  Engine  Foundations 

Foundation  piers  for  additional  supports  under  large  or  heavily  loaded 
buildings  are  constructed  in  the  same  manner  as  simple  foundations,  the 
size  being  determined  by  the  estimated  load  and  the  character  of  the 
ground.  The  footing  is  important,  as  the  sole  object  of  such  construction 
is  distribution  of  the  load. 

Foundations  for  gasolene  or  steam  engines  and  for  any  machinery  sub- 
ject to  considerable  vibration  are  constructed  in  the  same  manner  as 
foundation  piers.  The  size  and  depth  are  determined  by  the  amount  of 
vibration  to  be  withstood.  The  problem  is  simply  to  build  in  the  earth 
a  solid  block  of  concrete  of  weight  sufficient  to  withstand  the  action  of  the 
engine  bolted  to  its  top.  Casings  for  the  bolts  are  made  of  2-inch  pipe, 
resting  on  plates  at  the  lower  end  of  the  bolts;  they  are  embedded  in  the 
concrete  and  provide  for  any  necessary  adjustment  of  the  bolts  when 
setting  the  engine  in  place.  The  length  of  each  casing  equals  the  length 

36 


of  the  bolt  to  be  embedded;  by  tightening  the  nut  on  each  bolt  above  the 
templet  the  casing  fits  snugly  against  the  templet,  and  the  top  of  the 
bolt  is  brought  to  proper  height.  The  templet  can  be  made  from  1-inch 
material,  and  will  be  sufficient  for  placing  the  casings  in  smaller  engine 
foundations;  for  larger  foundations  cross-bracing  should  be  added.  In 
setting  bolts  first  nail  the  templet  securely  in  place,  then  mark  accurately 
the  position  of  the  bolts,  and  bore  holes  only  slightly  larger  than  the  bolts. 
Be  sure  the  bolt  holes  are  correctly  located.  Bolts  and  casings  are  now  set 
in  place,  centering  casings  with  bolts  by  several  nails  or  by  wooden  strips 
lightly  nailed  on  the  under  side  of  the  templets.  Proportions  of  1:3:5 
may  be  used  in  foundations  for  gasolene  engines  and  cream  separators. 
A  mixture  of  1:2:4,  using  aggregate  up  to  2  inches  or  2J^  inches,  is  recom- 
mended for  steam  engines  and  large  machinery. 


II.  Walls 
Cellar  and  Basement  Walls 

Wherever  the  excavation  is  made  for  a  cellar  under  a  building,  the 
problem  includes  not  only  the  construction  of  a  wall  to  serve  the  purpose 
of  a  foundation  for  the  superstructure,  but  of  one  which  will  also  insure  a 
cellar  warm  in  winter,  cool  in  summer,  and  dry  at  all  seasons.  Concrete 
walls  of  suitable  thickness  solve  the  problem  of  heat  transmission,  and  if 
properly  built,  the  cellar  will  be  always  dry.  Proper  drainage  should, 
however,  be  provided.  While  a  concrete  cellar  wall  may  be  constructed 
so  impermeable  that  water  standing  outside  will  not  penetrate  to  the 
interior,  drainage  to  natural  outlets  is  a  wise  precaution  and  should  not  be 
omitted  except  in  soil  that  is  dry  all  the  year  around. 

\Vhen  the  cellar  excavation  (often  made  by  team  and  scraper)  has  ir- 
regular sides  and  is  somewhat  larger  than  the  actual  dimensions  of  the 
wall,  it  will  be  necessary  to  use  both  outside  and  inside  wall  forms.  Only 
in  small  excavations  shoveled  by  hand  and  left  with  true  sides  in  firm  earth 
free  from  indications  of  caving  can  the  earth  be  used  for  the  outer  form.  . 

In  using  forms  for  both  the  outside  and  the  inside  of  the  wall  quite  a 
large  amount  of  lumber  would  be  required  if  forms  for  the  entire  work 
were  constructed  at  one  time.  To  obviate  this,  forms  can  be  built  in  sec- 
tions, each  section  being  of  the  full  height  of  the  cellar  wall,  and  as  long  as 
convenient  to  build  and  set  in  place.  An  entire  section  should  be  filled 
at  one  operation  in  order  to  avoid  horizontal  joints  or  lines  of  cleavage  in 
the  concrete. 

At  the  end  of  the  section  a  piece  of  2-inch  by  4-inch  lumber,  with  both 
edges  beveled  to  permit  of  easy  removal,  is  fastened  to  the  face  of  the  parti- 

37 


tion  board  used  as  a  stop-off  at  the  section's  end.  This  makes  a  tongue- 
and-groove  vertical  joint.  When  the  forms  are  ready  to  fill  for  the  adjoin- 
ing section,  the  end  of  the  partially  hardened  section  must  be  cleaned, 
wetted,  and  coated  with  neat  cement  grout  mixed  to  the  consistence  of 
thick  cream.  Attention  is  called  to  the  preference  in  building  practice  for 
vertical  joints  in  foundation  and  cellar  walls,  whereas  horizontal  joints  are 
preferable  in  the  upper  part  of  the  building. 

Sectional  forms  are  better  and  more  economically  constructed  by  build- 
ing them  flat  upon  the  ground  than  by  constructing  them  in  the  position 
in  which  they  are  to  be  used.  Care  should  be  exercised  to  build  them  true 
and  to  have  the  face  as  free  from  irregularities  as  possible.  The  sheeting 
for  the  inside  of  the  wall  should  be  surfaced  on  the  side  next  to  the  concrete, 
to  give  a  smooth  interior  finish. 

The  outer  and  inner  form  should  be  joined  at  the  top  by  nailing  cleats 
between  the  uprights,  being  careful  to  separate  the  forms  the  exact  width 
of  the  wall.  The  forms  should  be  united  a  short  distance  from  the  bottom 
by  double  wires,  and  should  be  separated  at  the  same  place  by  wood  spac- 
ing blocks  of  a  length  equal  to  the  thickness  of  the  wall.  When  the  spacing 
blocks  are  placed,  the  double  wires  are  twisted  by  the  use  of  a  large  nail,  so 
that  the  outer  and  inner  forms  are  firmly  fastened  together.  They  are  sup- 
ported by  securely  bracing  the  inner  form  so  that  the  wall  will  be  plumb. 

If  desired  to  provide  the  foundation  with  greater  resistance  to  lateral 
pressure,  or  to  afford  a  firmer  base,  "batter"  in  the  wall  may  be  secured  by 
lengthening  the  spacing  block  which  separates  the  outer  and  inner  forms. 

Anchor  bolts  are  embedded  in  the  top  of  the  concrete  at  suitable  inter- 
vals for  fastening  the  wall  plate  to  the  foundation. 

Cellar  Floors 

The  methods  of  building  concrete  walks  are  fully  described  in  another 
lesson.  The  methods  of  building  cellar  floors  are  similar.  To  avoid  repe- 
tition, only  the  points  of  dissimilarity  will  be  stated  here. 

Where  the  ground  is  firm  and  well  drained,  the  subbase  may  be  omitted 
and  the  concrete  floor  laid  directly  on  the  ground. 

Drainage  should  be  provided,  preferably  toward  the  center  of  the  floor. 
The  top  of  the  floor  should  be  given  grade  enough  that  water  accumulating 
from  scrubbing  or  other  causes  will  run  off  through  a  tile  drain  laid  beneath 
the  floor  and  communicating  with  a  natural  outlet. 

Where  a  basement  floor  is  below  the  level  of  ground  water,  the  floor 
should  be  laid  in  a  single  sheet  instead  of  being  divided  into  slabs.  The 
concrete  should  be  mixed  in  the  proportions  of  1:2:3,  and  the  floor  rein- 
forced in  both  directions  with  J^-inch  rods  8  inches  apart,  or  by  wire  mesh 
having  an  equal  cross-sectional  area  of  metal. 

38 


Entrances 

Outside  entrances  to  cellars  should  be  constructed  by  building,  at  right 
angles  to  the  cellar  wall,  forms  for  side  walls  sloping  from  the  top  of  the 
foundation  down  to  the  ground  and  from  the  cellar  floor  up  to  the  top  of 
the  proposed  stairway.  If  excavation  is  carefully  done,  the  earth  may 
usually  be  used  for  the  outer  form.  By  pouring  one  side  wall  at  a  time, 
and  reversing  the  form  by  changing  uprights  to  the  other  side  of  the  sheet- 
ing, one  form  may  be  used  for  both  sides  of  the  entrance.  The  form  for 
the  steps  may  be  built  after  the  side  walls  are  hard  enough  to  remove  the 
forms.  After  the  desired  measurements  of  tread  and  riser  have  been 
decided  upon,  the  plan  should  be  laid  out  on  the  side  walls,  cross-pieces 
wedged  between  them  and  secured  by  bracing.  The  concrete  used  in  the 
construction  of  the  base  should  be  as  wet  as  possible  without  flowing  from 
one  step  to  another. 

The  %-inch  facing  course  of  the  risers  may  be  placed  either  by  using 
a  thin  metal  partition  or  by  plastering  the  mortar  on  the  inside  of  the  face 
form  before  placing  the  coarse,  wet  concrete.  The  wearing  course  of 
treads  is  placed  as  in  sidewalk  work,  and  should  be  finished  by  wooden 
float  to  a  surface  reasonably  smooth  but  rough  enough  to  afford  a  good 
foothold. 

Window-frames 

Closer  joints  will  be  secured  under  cellar  windows  if  the  frames  are  not 
placed  until  the  concrete  has  hardened.  Extreme  care  should  be  taken  to 
have  the  opening  true,  thus  simplifying  the  work  of  placing  the  frames  and 
making  the  joints  tight. 

Finish 

Concrete  for  cellar  walls  should  be  of  such  consistence  that  when  poured 
into  the  forms  it  will  settle  to  place  by  gravity.  While  the  forms  are  being 
filled  the  coarser  aggregate  should  be  spaded  away  from  the  face  of  the 
wall,  bringing  the  mortar  next  to  the  forms.  The  mixture  recommended 
for  foundations  and  basement  walls,  1:3:5,  provides  an  excess  of  mortar 
for  this  purpose.  Spading  is  equally  important  on  the  interior  and  the 
exterior.  On  the  interior  it  gives  a  more  finished  surface,  and  on  the 
exterior  it  increases  water-tightness.  On  the  outside  of  the  wall,  above 
ground  line,  the  plastic  appearance  which  walls  will  have  after  forms  are 
removed  may  be  overcome  by  removing  the  surface  film  of  mortar  by 
brushing  with  a  wire  or  a  stiff  fiber  brush  and  washing  the  wall  with  the 
acid  solution  mentioned  in  the  lesson  "The  Surface  Finish  of  Concrete." 

39 


Removal  of  Forms 

Not  only  the  proportions  of  ingredients  and  consistence  of  concrete 
itself,  but  the  weather  conditions  have  marked  influence  upon  the  time  of 
hardening.  Consequently  no  definite  rule  can  be  given  for  removal  of 
forms.  Two  to  three  weeks  will  suffice  under  average  conditions.  Where 
the  earth  is  utilized  for  the  outer  form  more  time  will  be  required  than 
where  both  forms  are  of  lumber.  Too  early  removal  spells  failure,  and 
judgment  must  be  exercised. 

Block  Foundation  Walls 

Well-made  concrete  blocks  are  extensively  used  for  foundation  and 
cellar  walls.  For  the  latter  purpose  they  possess  the  advantage  of  an 
interior  air-space  which  helps  to  preserve  an  even  temperature  in  the 
cellar.  Care  should  be  exercised  that  the  blocks  are  well  made  of  properly 
selected  and  proportioned  materials,  mixed  wet  enough  so  that  the  per- 
centage of  porosity  and  absorption  will  be  low.  For  both  foundation  and 
cellar  walls  blocks  must  invariably  be  laid  in  cement  mortar  mixed  in  the 
proportion  of  1  sack  of  Portland  cement  to  2  cubic  feet  of  sand,  and  the 
joints  must  be  thoroughly  filled.  For  the  even  and  correct  filling  of  joints, 
a  templet  or  mortar  gage  may  be  obtained  from  the  manufacturers  of  lead- 
ing block  machines. 

Walls  for  Superstructures 

The  recent  statement  that  the  annual  fire  loss  of  American  farm  build- 
ings equals  one-fourth  of  their  total  cost  should  be  sufficient  argument  for 
concrete — a  material  that  will  not  burn. 

There  are  several  methods  of  using  concrete  for  the  main  portion  of  all 
classes  of  buildings.  The  most  common  forms  of  its  application  are  con- 
crete block  and  monolithic  walls.  The  concrete  block  is  fully  discussed 
in  another  lesson.  Monolithic  walls  may  be  either  plain  or  reinforced. 
The  principal  reason  for  reinforcing  monolithic  concrete  walls  is  to  prevent 
cracks  from  the  expansion  and  contraction  of  the  concrete  caused  by 
changes  in  temperature.  All  walls  exceeding  12  feet  in  height  should  be 
protected  by  sufficient  horizontal  and  vertical  reinforcement,  which  should 
depend  upon  dimensions  and  design  of  the  particular  structure.  It  is 
seldom  necessary  to  reinforce  monolithic  walls  over  8  inches  in  thickness 
when  less  than  12  feet  in  height,  except  around  window  and  door  openings. 
One-quarter-inch  rods  should  be  placed  from  1  inch  to  2  inches  back  from 
the  surface  of  the  wall,  and  2  inches  from  the  angles  of  openings;  three 
rods  above  and  two  on  each  side  of  the  openings;  two  rods  below  windows; 
all  projecting  10  inches  beyond  point  of  intersection.  Diagonal  rods  2}/2  feet 
long  should  be  placed  to  pass  intersections  of  horizontal  and  vertical  rods. 

40 


The  monolithic  concrete  wall  lends  itself  more  readily  than  any  other 
type  of  building  construction  to  the  individual  taste  of  the  builder  as  to 
variety  of  design.  In  this  respect  it  has  no  limitation  except  that  of  the 
builder's  ingenuity  in  the  construction  of  forms.  Forms  for  walls  above 
ground  must  necessarily  be  more  carefully  constructed  than  those  for 
cellar  work,  as  more  perfect  alinement  is  required  and  better  surface  finish 
desired.  Home-made  forms  are,  however,  often  used,  being  built  from 
2-inch  plank  surfaced  on  one  side,  braced  within  and  without  and  tied  to- 
gether in  the  manner  already  described  for  cellar  walls.  In  building  walls 
above  ground  level  continuous  forms  are  sometimes  used  to  avoid  vertical 
joints. 

Several  systems  of  clamps  are  now  manufactured  for  constructing 
forms  of  2-inch  plank.  They  generally  provide  for  courses  24  inches  in 
height,  the  same  form  being  moved  upward  as  soon  as  the  last  course  has 
hardened  sufficiently,  thus  effecting  a  great  saving  in  lumber,  although 
requiring  a  little  more  time  in  building. 

Metal  forms  are  now  obtainable  and  are  in  use  by  numerous  contractors. 
They  are  serviceable  and  satisfactory  and  may  be  rented  by  the  individual 
user  from  many  of  the  manufacturers. 

REFERENCES 

"Reinforced  Concrete  Construction,"  by  George  A.  Hool.     Published  by  McGraw-Hill 

Book  Company,  New  York  City. 
"Concrete  Plain  and  Reinforced,"  by  Taylor  and  Thompson.     Published  by  John 

Wiley  and  Sons,  New  York  City. 


VI.  The  Surface  Finish  of  Concrete 

CONCRETE  is  a  product  resulting  from  scientifically  combining  certain 
ingredients  to  form  a  material  useful  in  construction  because  of  its  own  dis- 
tinctive merits  and  not  because  of  its  resemblance  to  any  other  natural  or 
artificial  product.  When  properly  treated,  it  develops  beauty;  it  is  not 
the  beauty  of  onyx,  marble,  or  granite,  but  the  beauty  of  concrete.  If 
taken  for  what  it  is,  rather  than  what  it  resembles,  its  qualities,  uses,  and 
advantages  are  found  worthy  of  exhaustive  research. 

Mortar  Facing 

Whenever  it  is  desired  to  secure  a  rich  mortar  surface,  one  of  three 
methods  is  employed.  A  mortar  mixed  to  the  consistence  of  paste  may  be 
spread  on  the  inside  of  the  forms  and  the  concrete  filled  in  behind  it  and 
all  tamped  at  one  operation,  to  secure  a  good  bond.  A  board  or  block  of 

41 


the  desired  thickness  may  be  inserted,  the  concrete  filled  in,  and  the  board 
removed,  leaving  a  space  to  be  filled  by  the  mortar,  using  in  this  case  a 
slightly  wetter  mixture.  The  third  method,  and  by  far  the  most  general 
practice,  consists  in  using  a  partition  of  sheet  iron  or  steel  having  angle 
iron  attached  to  one  side  to  gage  the  thickness  of  facing  mortar.  Handles 
are  attached  to  the  top  so  that,  as  the  face  mortar  is  placed  in  front  first 
and  the  concrete  behind,  the  partition  is  gradually  moved  upward.  The 
use  of  a  rich  mortar  is  not  so  prevalent  as  it  was  in  the  earlier  stage  of  the 
concrete  industry,  because,  as  will  be  explained,  far  more  pleasing  effects 
may  now  be  secured  by  other  methods. 

Spading 

Where  a  smooth  concrete  surface  is  desired,  a  spade  or  face  cutter  is 
used,  which  is  forced  down  beside  the  forms  while  the  concrete  is  being 
placed,  forcing  the  coarse  aggregate  back  and  allowing  the  mortar  to  fill  the 
spaces  next  to  the  forms,  resulting  in  a  surface  as  smooth  as  the  face  of  the 
forms. 

Sand  Rubbed  Surfaces 

Probably  the  cheapest  method  of  finishing  a  smooth  concrete  surface 
consists  in  removing  the  forms  at  the  end  of  a  period  varying  from  six 
hours  to  three  days,  according  to  the  weather  conditions,  and  finishing  the 
surface  by  the  use  of  a  plasterer's  float  or  small  board,  using  sand  and 
plenty  of  water  between  the  board  and  the  wall  to  do  the  cutting.  If  this 
work  is  done  at  a  time  when  the  concrete  is  neither  too  green  nor  too  hard, 
good  results  can  be  cheaply  secured,  as  a  laborer  will  in  this  manner  cover 
100  square  feet  in  an  hour.  This  method  of  finishing  is  recommended  for 
factory  construction  and  the  rear  of  apartment  buildings,  and  in  general 
for  such  walls  as  do  not  require  special  treatment. 

Experimentation 

Careful  trials  should  be  made  before  undertaking  any  artistic  treat- 
ment upon  actual  construction  work.  The  possibilities  and  variations  in 
this  work  are  unlimited,  and  some  methods  of  work  are  expensive.  Conse- 
quently any  one  intending  to  attempt  the  construction  of  an  artistic  surface 
should  first  try  out  the  proposed  method  on  several  small  samples  from  6 
to  12  inches  square. 

The  surface  desired  will  determine  the  selection,  gradation,  and  pro- 
portioning of  the 'aggregate,  and  will  also  influence  the  consistence  of  the 
mixture.  Some  of  the  more  common  materials  selected  for  aggregate  will 
be  limestone,  granite,  marble  chips,  and  other  sto*^  and  gravels  and  sands 
of  various  colors. 

42 


Whenever  it  is  necessary  to  use  expensive  materials  to  obtain  the  sur- 
face finish  desired,  they  are  used  only  in  the  mixture  applied  as  a  facing  for 
surfaces  to  be  exposed.  As  a  rule,  the  facing  mixture  varies  from  1  to  1)^ 
inches  in  thickness,  the  remainder  of  the  work  being  of  ordinary  concrete. 
However,  both  must  be  placed  in  the  forms  at  the  same  time  to  insure  a 
perfect  bond  and  a  solid  mass.  The  third  method,  already  described,  for 
placing  mortar  on  the  face  of  concrete  work  is  recommended  in  this  con- 
nection; that  is,  the  use  of  a  partition  of  sheet  iron  or  steel.  As  rich  mortars 
always  have  a  tendency  to  develop  minute  cracks,  they  should  be  avoided, 
so  far  as  possible,  and  a  mixture  of  1  sack  of  Portland  cement  to  2}/£ 
cubic  feet  of  aggregate  is  therefore  recommended  in  the  production  of 
artistic  concrete  surfaces.  The  thickness  of  the  facing  material  should 
not  be  less  than-  one  inch  when  fine  aggregate  is  used,  and  whenever  coarse 
aggregate  is  used  it  should  be  at  least  twice  as  thick  as  the  greatest  diameter 
of  the  largest  aggregate  used. 

Brushed  Surfaces 

It  is  sometimes  desirable  to  remove  the  plaster-like  appearance  of  the 
concrete  as  it  comes  from  the  forms.  One  of  the  best  methods  of  over- 
coming this  is  to  remove  the  forms  in  about  twelve  hours  (it  being  always 
understood  that  the  time  of  removal  is  dependent  upon  the  weather  and  the 
nature  of  the  construction).  As  soon  as  the  forms  have  been  removed,  if 
a  brushed  surface  is  desired,  the  concrete  should  be  brushed  while  still 
green  with  a  steel  brush  or  one  of  stiff  palmetto  or  other  fiber  bristles.  A 
good  brush  may  also  be  made  by  clamping  together  enough  sheets  of  wire 
cloth  to  make  a  brush  about  four  inches  wide.  If  the  concrete  hardens  so 
that  the  mortar  cannot  be  brushed  away  from  the  coarse  aggregate,  the 
mortar  may  be  softened  by  a  solution  of  muriatic  acid.  After  brushing,  the 
work  should  be  treated  with  an  acid  solution,  and  for  this  purpose,  if 
standard  Portland  cement  has  been  used,  the  solution  should  be  one  part  of 
commercial  muriatic  acid  to  three  parts  of  water.  After  the  use  of  an  acid 
solution  the  work  should  be  washed  immediately  and  thoroughly  with  clean 
water,  as  any  acid  remaining  upon  the  face  of  the  work  will  ultimately 
cause  streaks  and  discoloration. 

The  following  materials  are  recommended  as  suitable  aggregates  for 
the  production  of  desirable  brushed  surfaces,  it  being  understood  in  using 
any  of  them  for  aggregates  that  the  mixture  is  to  be  1  sack  of  Portland 
cement  to  2^  cubic  feet  of  aggregate : 

Yellow  marble  screenings  up  to  J4  inch;  red  granite  screenings  up  to 
%  inch;  black  marble  graded  from  Y%  inch  to  Yi  inch;  white  marble  graded 
from  Y%  inch  to  Yi  inch;  river  or  lake  gravel  graded  from  34  inch  to  Yi  inch. 

43 


To  secure  economy,  limestone  may  be  substituted  for  white  marble,  and 
either  black  granite  or  trap-rock  may  be  substituted  for  black  marble. 

The  above  materials  are  merely  suggestive  of  the  possibilities  of  con- 
crete surfaces.  Infinite  variations  may  be  made  by  substituting  and  com- 
bining materials,  while  if  one  takes  trap-rock,  red  granite,  and  limestone, 
for  instance,  by  merely  increasing  or  diminishing  the  size  of  one  or  two  of 
the  ingredients  it  readily  will  be  seen  that  a  great  rrlany  combinations  may 
be  effected,  all  of  which  will  produce  desirable  surfaces  for  brushing.  In 
general,  fine  aggregate  will  produce  a  comparatively  smooth  surface  of  uni- 
form color,  while  coarser  aggregates  will  give  greater  irregularity  in  both 
surface  and  color,  producing  a  somewhat  rustic  appearance. 

One  of  the  chief  advantages  of  finishing  surfaces  by  brushing  is  the 
adaptability  of  this  process  to  every  class  of  concrete  construction.  Park 
benches,  lawn  vases,  lamp-posts,  and  statuary  of  all  kinds  may  be  finished 
by  this  process  as  easily  as  buildings. 

Rubbed  Surfaces 

Where  it  is  desired  to  leave  a  smooth  surface  in  the  shape  produced  by 
the  forms,  but  to  obtain  a  more  finished  surface  than  possible  by  washing 
with  a  float  under  which  sand  is  used  for  cutting,  the  concrete  may  be 
finished  when  it  is  at  an  age  of  from  one  to  two  days  by  removing  the  form 
and  rubbing  the  surface  with  a  brick  and  sand,  natural  stone,  emery,  or 
carborundum. 

Where  it  is  desired  to  finish  concrete  in  this  manner  the  large  pieces  of 
aggregate  should  be  spaded  back  from  the  forms  so  that  the  face  will  con- 
tain little  or  no  coarse  aggregate.  If  a  mottled  surface  is  desired,  it  may  be 
produced  by  a  mortar  composed  of  one  part  of  Portland  cement  and  2^ 
parts  of  white  marble  or  limestone,  either  of  which  will  rub  to  a  very  beauti- 
ful surface.  While  the  rubbing  is  in  process,  a  thin  grout  composed  of  one 
part  of  cement  and  one  of  sand  should  be  applied  and  well  rubbed  in.  The 
work  should  afterward  be  washed  down  with  clean  water. 

Dressed  Surfaces 

When  concrete  has  thoroughly  hardened,  it  may  be  dressed  in  the  same 
manner  as  natural  stone,  although  the  stone-cutter's  tools  require  slight 
alterations  to  suit  the  need  of  the  concrete.  While  this  work  is  sometimes 
done  upon  concrete  when  it  is  two  or  three  days  old,  the  best  results  are 
obtained  after  it  is  about  a  month  old.  The  great  disadvantage  of  dress- 
ing concrete  with  a  stone  hammer  at  too  early  an  age  is  that  pieces  of  the 
aggregate  will  be  knocked  out  from  the  cement  mortar,  leaving  unsightly 
holes,  while  if  left  for  a  few  weeks,  they  will  become  so  thoroughly  bonded 

44 


that  they  will  break  under  the  hammer  and  give  a  uniform  surface,  much 
the  same  as  natural  stone. 

For  this  purpose  the  best  tool  is  a  special  form  of  bush  hammer  designed 
to  dress  concrete,  the  points  on  the  face  of  which  are  farther  apart  and 
larger  than  on  the  regular  stone-cutter's  hammer.  A  three-pound  hammer 
with  four  points  is  a  good  size  for  concrete  work,  although  larger  ones  are 
frequently  used.  Another  hammer  which  has  been  especially  designed  for 
dressing  concrete  is  similar  to  a  pick  having  five  teeth  on  each  end.  This 
is  made  in  two  forms,  one  consisting  of  a  steel  head  six  inches  long,  beveled 
at  both  ends,  the  other  being  in  the  form  of  a  central  cast  steel  head  to 
which  steel  plates  are  bolted.  In  the  latter  form  the  plates  are  removable, 
and  when  dull  are  replaced  by  sharp  ones.  Three-eighths  inch  crushed 
granite  screenings  were  used  for  facing  the  exposed  surface  of  the  Connect- 
icut Avenue  Bridge,  Washington,  D.  C.,  and  the  finish  was  obtained  by 
bush  hammering.  Very  desirable  exteriors  may  be  produced  by  bush- 
hammered  panels  finished  with  2-inch  smooth  borders,  as  shown  on  the 
Piqua  Hosiery  Company's  Building,  Piqua,  Ohio,  a  replica  of  which  was 
exhibited  at  the  Cement  Show  in  1914.  By  using  this  method  all  trouble 
in  finishing  corners  is  eliminated,  and  the  architectural  design  accentuated 
and  improved.  For  finishing  large  surfaces  a  pneumatic  hammer  is  used, 
and  produces  a  very  uniform  finish,  doing  the  work  much  more  rapidly 
than  where  the  tools  are  operated  by  hand. 

Sand  Blast  Surfacing 

Sand  blast  is  frequently  used  for  finishing  concrete  surfaces  on  large 
construction.  It  removes  the  plaster  effect  left  by  the  forms  and  produces  a 
granular  finish.  Sand  blasting  involves  the  erection  of  quite  a  large  and  ex- 
pensive machine,  forcing  sand  grains  from  a  nozle  by  pneumatic  pressure 
and  driving  them  against  the  surface  of  the  wall  with  such  violence  that  the 
sand  cuts  out  the  softer  particles  of  the  concrete  against  which  it  is  thrown. 

Upon  a  dense  and  thoroughly  hardened  surface  a  ^-inch  nozle  may  be 
used,  but  if  the  surface  is  not  thoroughly  hard,  say  two  or  three  months 
old,  it  is  better  to  use  a  ^-  or  even  J'g-inch  nozle.  Crushed  quartz  or  sharp 
silica  sand  should  be  used  for  sand  blasting.  If  a  J^-inch  nozle  be  used,  the 
sand  should  be  screened  through  a  No.  8  screen;  if  a  J^-inch  nozle  is  used, 
the  sand  should  be  screened  through  a  No.  12  screen.  Concrete  should 
never  be  subjected  to  sand  blasting  until  it  is  at  least  one  month  old.  A 
nozle  pressure  of  from  50  to  80  pounds  should  be  maintained. 

Colored  Surfaces 

For  artistic  work  the  suggestions  already  made  with  reference  to  the 
selection,  gradation,  and  mixing  of  aggregate  will  accomplish  better  results 

45 


than  any  process  of  artificial  coloring  which  may  be  adopted.  However, 
this  paper  would  be  incomplete  if  some  information  were  not  included  re- 
garding the  possibilities  of  producing  artificially  colored  concrete  work. 

The  coloring-matter  should  never  exceed  5  per  cent,  of  the  weight  of 
the  cement,  and  should  be  mixed  with  the  dry  cement  before  water  is  added. 
Nothing  but  mineral  coloring-matter  should  be  used,  and  the  following 
table  gives  the  amounts  of  different  coloring-materials  to  be  employed. 


TABLE  OF  COLORS  TO  BE  USED  IN  PORTLAND  CEMENT 


COLOR  DESIRED 

COMMERCIAL  NAMES  OP  COLORS 
FOR  USE  IN  CEMENT 

APPROX- 
IMATE 
PRICES  PER 
LB.  IN  100- 
LB.  LOTS 
FOR  HIGH- 
GRADE 
COLORS 

POUNDS  OF  COLOR 
REQUIRED  FOR  EACH 
BAG  OF  CEMENT  TO 
SECURE  — 

Light 

Shade 

Medium 
Shade 

Grays,  blue-black  and  black 
Blue  shade  

f  Germantown  lampblack 
I  Carbon  black 
I  Black  oxide  of  manga- 
nese 
Ultramarine  blue 

Red  oxide  of  iron 
Mineral  turkey  red 

Indian  red 
Metallic  brown  (oxide) 

Yellow  ochre 
Chromium  oxide 

10  Cts. 

8    " 

6    " 
18    " 

3    " 

15    " 

10    " 
4    " 

6    " 
26    " 

« 
X 

1 
5 

5 
5 

5 
5 

5 
5 

1 
1 

2 
10 

10 
10 

10 
10 

10 
10 

Brownish-red  to  dull  brick 
red  

Bright  red  to  vermilion  
Red  sandstone  to  purplish- 
red  

Brown  to  reddish-brown  .... 
Buff,  colonial  tint,  and  yel- 
low   

Green  shade  

There  will  be  exceptional  cases  where  it  is  necessary  or  desirable  to 
color  concrete  surfaces  after  the  work  has  been  completed.  F6r  this 
purpose  cement  paint  should  be  used,  several  brands  of  which  are  now 
manufactured  in  a  limited  number  of  colors  by  reputable  companies. 

Designs 

There  remains  only  one  feature  of  concrete  surfaces  to  be  discussed, 
and  that  is  the  production  of  mosaics  or  pattern  work.  The  plasticity  of 
concrete  makes  it  lend  itself  particularly  to  the  reproduction  of  beautiful 
designs  of  all  sorts,  which  may  be  secured  in  a  variety  of  ways.  For  the 
more  elaborate  designs  the  pieces  of  marble,  if  that  be  the  material  selected, 
should  be  glued  face  down  upon  tough  paper  in  the  same  manner  in  which 
floor  tile  are  prepared  for  laying.  This  paper,  with  the  design  upon  it, 
should  be  placed  in  the  form,  and  the  concrete  filled  in  and  thoroughly 
rammed  to  place.  After  the  forms  are  removed  and  the  concrete  allowed 

46 


to  harden,  the  paper  should  be  removed  by  wetting;  then  clean  the  face 
of  the  finished  design  with  the  usual  acid  solution,  3  parts  of  water  to  1 
part  of  commercial  muriatic  acid. 


VII.  Cement  Products 

I.  Concrete  Blocks 
Historical 

The  use  of  concrete  blocks  is  of  ancient  origin,  and  although  it  is  the 
purpose  of  this  lesson  to  deal  essentially  with  modern  practice,  it  will  be 
interesting  to  know  that  blocks  of  various  forms  and  sizes  composed  of 
material  similar  to  the  concrete  of  to-day  were  used  in  many  of  the  monu- 
mental works  of  the  ancient  world,  and  may  still  be  seen  in  southern  Europe, 
well  preserved  after  the  lapse  of  centuries. 

The  introduction  of  concrete  blocks  in  America  was,  like  all  other  uses 
to  which  concrete  has  been  adapted,  coincident  with  the  perfection  of  the 
rotary  kiln  and  the  consequent  development  of  American  Portland  cement 
manufacture.  Houses  constructed  of  solid  blocks  or  of  blocks  separated 
by  metal  anchors  and  thus  forming  a  hollow  wall  may  still  be  seen  after  a 
half-century  of  usefulness;  but  these  are  examples  built  in  the  infancy  of  an 
industry  which  began  to  come  into  its  own  only  ten  years  ago.  At  that 
time  the  idea  of  making  blocks  in  shapes  designed  for  use  in  constructing 
hollow  walls,  to  insure  warmth  in  winter  and  coolness  in  summer, — blocks 
impervious  to  moisture  and  of  such  weight  that  they  could  be  readily 
made  and  laid, — gained  so  strong  a  hold  upon  the  popular  mind  that  many 
people  rushed  into  the  manufacture  of  concrete  blocks  without  adequate 
knowledge  of  the  nature  of  cement  or  of  the  methods  necessary  to  insure 
success  in  any  branch  of  the  concrete  industry.  Fortunately,  these  condi- 
tions have  corrected  themselves,  and  the  elimination  of  the  ignorant  and 
unscrupulous  block-maker  will  follow.  To-day  the  concrete  block  industry 
stands  upon  a  firm  foundation  of  experience  and  reliability,  the  compara- 
tively new  building  material  being  one  which,  having  proved  its  efficiency, 
has  come  to  stay. 

Utility 

The  fire-resisting  qualities  of  concrete  are  so  well  known  that  it  is 
necessary  only  to  call  attention  to  the  fact  that  the  design  of  the  concrete 
block  excels  all  other  forms  of  concrete  in  this  respect  (excepting  double 
monolithic  walls  only),  because  of  the  vertical  and  horizontal  air-spaces 

47 


within  the  wall,  which  so  far  prevent  the  transmission  of  heat  that  in 
numerous  fires  it  has  been  observed  by  reputable  witnesses  that  the  exposure 
of  one  side  of  a  well-built  12-inch  concrete  wall  to  an  intense  and  prolonged 
heat  did  not  even  damage  merchandise  on  the  other  side.  Damage  to  the 
blocks  themselves  has  usually  amounted  only  to  slight  chipping,  due  to  the 
dehydration  of  the  outer  part,  or  facing,  often  applied  in  making  blocks. 

The  same  feature,  namely,  the  non-conductivity  of  an  interior  air- 
space, results  in  a  decided  saving  of  fuel  during  the  winter  and  increased 
comfort  in  summer,  while  it  especially  distinguishes  the  concrete  block  as 
the  most  suitable  building  material  for  tropical  and  semi-tropical  climates. 

While  it  is  not  only  possible,  but  commercially  practicable,  to  make 
concrete  blocks  of  water-tight  texture,  the  interior  air-space  makes  "assur- 
ance doubly  sure"  during  protracted  rainy  spells,  and  effectually  safe- 
guards from  the  sweating  so  objectionable  in  other  types  of  construction. 
Wherever  the  type  of  block  used  affords  a  continuous  horizontal  and  vertical 
air-space,  as  in  two-piece  walls,  furring  may  be  eliminated. 

Materials  of  Manufacture 

The  first  requisite  to  the  manufacture  of  a  good  concrete  block  is  suit- 
able materials.  The  concrete  block  is  a  composite  product,  and  can  be 
no  better  than  its  weakest  ingredient. 

The  fundamental  requirement  is  high-grade  Portland  cement,  which 
must  be  kept  in  dry  storage  until  used.  Cement  which  has  become  damp 
enough  to  harden  must  never  be  used  in  making  concrete  blocks.  If  pro- 
portioning is  done  by  volume,  the  commercial  sack  of  Portland  cement  may 
be  accepted  as  one  cubic  foot.  If,  however,  sacks  are  opened  before  pro- 
portioning, the  cement  increases  in  bulk  so  materially  that  it  is  necessary 
to  proportion  by  weight.  (A  sack  contains  94  pounds  net.) 

Sand 

The  sand  used  in  the  mortar  of  a  concrete  block  should  be  silicious, 
coarse  and  clean.  If  screened  from  bank-run  gravel,  care  should  be  exer- 
cised to  see  that  it  is  free  from  animal  or  vegetable  matter.  If  foreign 
substances  are  present,  they  should  be  removed  by  washing  the  sand.  The 
selection  of  aggregates  and  methods  of  determining  their  suitability  have 
been  given  in  a  previous  lesson. 

Gravel  or  Broken  Stone 

The  coarse  aggregate  for  concrete  blocks  should  consist  of  gravel  or 
broken  stone.  Choice  between  the  two  depends  upon  local  availability 
and  desired  surface  finish.  Crusher-run  broken  stone  should  not  be  used 
until  the  dust  has  been  screened  out  and  the  stone  properly  sized  for  pro- 

48 


portioning.  Bank-run  gravel  must  be  screened  and  reproportioned  before 
using.  This  point  cannot  be  too  strongly  emphasized,  as  many  failures 
are  directly  attributable  to  neglect  of  this  requirement.  Whether  strength 
and  density  or  economy  and  the  saving  of  cement  be  the  aim,  the  block- 
maker  cannot  afford  to  use  unscreened  bank-run  gravel. 

So  far  as  strength  is  concerned,  it  is  impossible  to  make  a  concrete 
block  stronger  than  the  aggregate  of  which  it  is  in  part  composed.  The 
ultimate  strength  is  demonstrated  when  a  fractured  block  shows  the  cleav- 
age— and  not  the  pulling  apart — of  the  coarse  aggregate. 

As  to  surface  finish,  the  possible  variations  resulting  from  choice  of 
aggregate  are  numerous.  The  granites,  marbles,  white  quartz,  and  gravel 
of  variegated  colors  are  increasingly  popular  for  exposed  surfaces.  For  the 
main  portion  of  the  block,  necessarily  cheaper,  limestone  is  in  most  local- 
ities the  best  available  broken-stone  aggregate.  Sandstones  are  variable 
in  strength,  and  the  softer  ones  do  not  make  good  concrete  blocks.  Hard, 
clean  gravel  is  often  cheaper  than  broken  stone  and  is  equally  desirable. 

Proportioning 

When  the  materials  have  been  selected,  the  next  step  will  be  their 
proper  proportioning,  and  for  this  purpose  it  is  necessary  to  establish  an 
arbitrary  standard  of  sizes — the  sand  grains  passing  through  a  screen  of  J£- 
inch  mesh,  and  gravel  or  broken  stone  passing  a  1-inch  ring  and  being 
retained  on  a  sand  screen. 

There  are  certain  well-established  scientific  methods  of  determining 
voids  and  establishing  definite  proportions,  which  has  been  fully  treated  in 
a  previous  lesson.  For  the  present  it  may  be  stated  that  under  average 
normal  conditions  a  mixture  of  1  part  Portland  cement,  2J^  parts  of  sand, 
and  4  parts  of  gravel  or  broken  stone  (expressed  as  1 :  2J/£ :  4)  has  been  found 
most  satisfactory  for  the  body  or  main  portion  of  all  blocks.  If  it  is  desired 
to  face  the  block  with  a  finer  material,  the  richness  will  be  increased  in  pro- 
portion to  the  elimination  of  coarse  aggregate,  but  1 :1)^  is  as  rich  as  should 
be  used  for  any  face,  while  a  1 : 2  is  better  except  in  cases  where  decidedly 
fine  texture,  such  as  tooled  and  scrolled  work,  is  desired. 

Mixing 

No  matter  how  carefully  the  materials  may  be  proportioned,  good  con- 
crete blocks  cannot  be  obtained  unless  the  mixing  is  properly  and  thoroughly 
done.  For  important  work  it  is  both  safer  and  cheaper  to  use  a  power- 
driven  mixer  of  standard  make  and  known  efficiency.  However,  where 
the  proposed  work  is  not  extensive  enough  to  warrant  the  installation  of  a 
mixer,  equally  good  results  can  be  obtained  from  painstaking  hand  mixing, 
using  a  water-tight  platform,  first  spreading  out  the  sand,  then  the  cement, 
4  49 


mixing  both  together  thoroughly,  then  adding  the  water  and  shoveling 
until  the  mortar  is  of  uniform  color;  after  this  the  coarse  aggregate,  which 
has  first  been  thoroughly  wetted,  should  be  added,  and  the  whole  mass 
turned  twice  after  its  addition. 

Consistence 

The  water  used  should  be  clean  and  used  in  such  quantity  that  a  me- 
dium wet  mixture  will  result.  By  this  is  meant  one  that  shows  rather  an 
excess  of  water,  so  that  when  a  small  portion  of  the  mass  is  firmly  pressed 
in  the  hand,  several  drops  of  water  will  be  released  from  the  concrete. 

No  other  consistence  of  mix  is  now  recommended,  because  the  dry  mix 
resulted  in  almost  certain  failure  and  the  flowing  mix  was  commercially 
impracticable  for  small  plants,  on  account  of  the  large  number  of  molds 
and  the  consequent  expense  of  equipment  required. 

Molds  and  Machines 

Blocks  are  made  by  tamping  or  pressing  the  concrete  in  molds  designed 
for  the  purpose,  and  it  is  manifestly  beyond  the  scope  of  this  lesson  to  dis- 
cuss the  various  machines  individually.  The  choice  of  a  machine  is,  in 
the  main,  a  matter  of  price,  stability  of  construction,  and  minor  details  of 
operation.  Most  machines  provide  for  a  block  of  convenient  size  and 
weight,  penetrated  by  cores  which,  when  withdrawn,  leave  the  hollow 
space  which  gives  the  concrete  block  its  peculiar  efficiency.  The  machines 
operated  by  pressure  instead  of  tamping  generally  make  the  two-piece 
blocks;  that  is,  the  blocks  do  not  extend  entirely  through  the  wall  as  do 
the  tamped  hollow  blocks.  Both  processes — tamping  and  pressing — and 
both  designs,  hollow  and  two-piece  blocks,  are  now  accepted  as  good  con- 
struction by  engineers  and  architects,  if  the  rules  heretofore  given  relative 
to  selection,  proportioning,  mixing,  and  consistence  are  observed.  If  they 
are  disregarded,  no  machine  can  produce  a  concrete  block  which  will  be 
creditable  to  the  maker  or  satisfactory  to  the  user. 

Curing 

The  curing  of  concrete  blocks  is  a  very  important  part  of  the  manu- 
facturing process.  The  setting  of  cement,  or  its  crystallization,  is  a  chem- 
ical reaction,  accelerated  by  heat  and  possible  only  in  the  presence  of 
moisture. 

If  cured  by  water,  blocks  should  remain  in  a  closed  room  for  twenty- 
four  hours,  after  which  they  may  be  stacked  under  a  shed  with  open  sides. 
Blocks  require  frequent  sprinkling  for  two  weeks,  and  are  not  ready  for 
use  until  a  month  old.  Moreover,  their  color  is  affected  by  the  variation 
of  moisture  and  heat  caused  by  wind  currents  to  which  they  are  necessarily 

50 


subjected  in  the  open-air  curing  shed.  To  overcome  these  objections  and 
to  shorten  the  curing  period,  we  strongly  urge,  wherever  possible,  the 
construction  of  a  closed  steam-curing  room,  in  which  the  blocks  may  be 
cured  for  forty-eight  hours  in  a  saturated  atmosphere  at  a  temperature  of 
100°  to  130°  Fahrenheit.  The  time  should  be  doubled  in  winter.  Such 
curing  will  be  more  effective,  as  the  blocks  will  develop  greater  strength 
in  ten  days  than  air-cured  blocks  will  in  twenty-eight  days.  The  color  will 
more  closely  approach  uniformity,  owing  to  the  fact  that  each  block  thus 
receives  the  same  treatment.  The  corners  and  facing  of  blocks  will  not 
be  exposed  to  the  usual  injuries  almost  inseparably  connected  with  setting 
green  blocks  in  the  yard.  The  saving  of  time  and  yard  room  is  by  no 
means  an  insignificant  item.  Steam  curing  makes  it  possible  to  operate 
the  plant  twelve  months  in  the  year. 

Building  Construction 

The  different  manufacturers  of  concrete  block  machines  have  evolved 
designs  for  corner,  jamb,  and  chimney  blocks  and  other  special  members, 
according  to  the  requirements  of  each  particular  system.  These  are  gen- 
erally well  adapted  to  their  intended  usages,  but  the  block-maker  must  bear 
in  mind  that  corners  and  jambs  are  subjected  to  greater  wear  and  greater 
possibility  of  accident  than  are  "stretcher"  blocks,  and  suffer  more  exposure 
in  time  of  fire.  Consequently  they  require  special  care  in  making  and  will 
cost  proportionately  more.  Accessories,  such  as  joist-hangers  at  floor 
levels  and  T-rods  for  securing  roof  plates,  are  manufactured  by  several 
reputable  firms  and  are  advertised  in  the  columns  of  current  concrete  pub- 
lications. 

Footings  should  be  of  poured  concrete  in  which  the  lower  course  of  the 
foundation  wall  may  be  embedded.  Concrete  blocks  12  inches  wide  form 
an  ideal  cellar  wall,  this  being  ample  thickness  for  the  foundation  wall  of  a 
two-story  building.  The  walls  of  the  first  story  may  be  of  the  same  width, 
those  of  the  second  story  reducing  to  10  inches.  Higher  buildings  will 
usually  be  constructed  in  cities  or  towns  where  thickness  of  walls  is  regulated 
by  ordinance. 

Appearance 

The  possible  variations  in  surface  finish  of  concrete  blocks  afford  almost 
unlimited  opportunity  to  the  block-maker  who  remembers  that  concrete 
is  a  separate  and  distinct  building  material,  possessing  possibilities  beyond 
the  range  of  those  afforded  by  either  brick  or  stone.  If  he  grasps  this  fact, 
he  will  cease  his  efforts  to  produce  a  plastic  and  unpleasing  counterfeit  of 
the  cheaper  grades  of  stone  work.  He  will  learn  that,  by  proper  selection 
of  aggregate,  he  can  secure  a  surface  which,  left  plain  and  smooth,  is  as 

51 


beautiful  as  a  mosaic,  or  which,  roughened  by  brushing  a  film  of  cement 
from  the  surface  of  a  newly  made  block  and  washing  the  face  so  roughened 
with  a  1:3  solution  of  commercial  muriatic  acid,  will  produce  effects  of 
startling  originality  and  beauty. 

II.  Concrete  Fence  Posts 

General  Requirements 

What  has  been  said  in  Section  I  of  this  lesson  regarding  the  general 
principles  of  concrete  construction  will  apply  with  equal  force  to  concrete 
fence  posts,  and  will  not,  therefore,  require  repetition. 

The  concrete  fence  post,  like  the  concrete  block,  is  a  comparatively 
small  unit,  manufactured  for  a  particular  purpose,  and  thoroughly  seasoned 
before  being  put  to  its  ultimate  use.  Consequently  the  same  care  must  be 
exercised  in  the  selection  as  well  as  in  the  proportioning  and  mixing  of 
materials. 

Consistence 

Slightly  more  water  is  necessary  in  mixing  concrete  for  fence  posts  than 
is  used  for  blocks,  owing  to  the  different  process  of  manufacture.  A 
quaky  mixture,  which  is  wet  enough  to  be  just  beyond  the  possibility  of 
tamping,  is  used  for  posts — compactness  in  filling  the  molds  being  secured 
either  by  agitating  the  concrete  by  stirring  or  jarring  the  mold. 

Reinforcement 

In  but  one  other  respect  does  the  concrete  fence  post  depart  from  the 
process  of  manufacture  applied  to  the  concrete  block.  The  peculiar  duties 
demanded  of  the  concrete  fence  post  subject  it  to  strains  beyond  the  lateral 
resistance  of  a  plain  concrete  member  having  such  a  small  cross-section. 

To  overcome  the  strains  and  thrusts  peculiar  to  the  duty  demanded  of 
the  fence  post  reinforcing  wires  or  rods  of  steel  are  introduced,  and  it  is 
very  important,  both  as  regards  the  strength  of  the  post  and  the  saving  of 
material,  that  the  reinforcement  be  properly  placed.  By  imagining  a  post 
constructed  of  rubber  and  considering  how  such  a  post  would  act  if  bent 
far  over  to  one  side,  the  theory  of  reinforcement  is  easily  pictured  to  the 
mind.  A  rubber  post  would  manifestly  be  stretched  on  one  side  and 
pinched  on  the  other,  so  we  say  that  one  side  of  the  post  will  sustain  tensile 
stress  while  the  other  will  be  subject  to  compression.  As  is  well  known, 
concrete  is  strong  in  compression  and  weak  in  tension,  or  resistance  to 
pulling  strains;  hence  on  the  side  that  is  stretched  we  introduce  just  steel 
enough  to  balance  or  develop  the  opposing  compressive  strength  of  the 

52 


concrete.  Thus  we  secure  maximum  efficiency  from  both  the  concrete  and 
the  steel.  We  are  unable  to  tell  in  advance  which  of  the  four  sides  of  a 
post  will  be  called  upon  to  withstand  the  thrust;  therefore,  we  usually 
embed  at  each  corner  of  the  post,  ^  inch  from  the  surface,  a  J^-inch  steel 
rod,  twisted  bar,  or  wire.  In  this  way  a  direct  load  from  any  side  is 
resisted  by  two  rods  acting  in  unison. 

Dimensions 

Posts  are  usually  made  7  feet  long,  3  inches  square  at  the  top,  and  5 
inches  square  at  the  bottom,  or  they  may  be  made  4  inches  square  at  the 
top  and  4  inches  by  6  inches  at  the  bottom.  The  dimensions  first  given 
are  usually  preferred,  on  account  of  the  taper  on  all  four  sides  making  it 
very  easy  to  fasten  the  line  wire  by  merely  tying  a  small  wire  to  it  and 
making  it  taut  around  the  post.  The  method  of  making  holes  through 
posts  is  objectionable  because  of  weakening  the  posts,  and  also  because 
such  holes  establish  an  arbitrary  place  for  fastening  line  wires  which  is  fre- 
quently inconvenient  and  often  interferes  with  uniformity  in  fence  con- 
struction. The  last-mentioned  objection  is  also  a  fault  of  staple  and  T- 
shaped  fastening  devices  of  metal,  which  are,  moreover,  liable  to  failure 
from  rusting  on  account  of  exposure  to  the  weather. 

Molds 

The  preparation  of  "knock-down"  molds  using  head  pieces  and  clamps 
— the  lumber  being  protected  from  warping  by  painting  with  equal  parts 
of  boiled  linseed  oil  and  kerosene — is  a  very  simple  matter,  and  will  cause 
no  inconvenience  to  the  ordinary  manufacturer.  However,  if  he  has  a 
large  amount  of  fence  to  build,  it  will  probably  be  more  profitable  for  him 
to  purchase  a  set  of  steel  molds  from  one  of  several  firms  now  manufactur- 
ing them  in  sets,  known  as  "gang"  molds,  which  permit  making  from  4 
to  12  posts  at  one  operation. 

Whether  a  mold  be  made  of  steel  or  lumber,  the  essential  points  are 
that  its  sides  shall  be  strong  enough  to  remain  true  under  the  lateral  pressure 
incident  to  filling  the  mold  compactly,  and  that  the  mold  be  so  constructed 
that  the  long,  slender  concrete  post  may  remain  undisturbed  until  it  has 
attained  sufficient  rigidity  to  be  removed  without  harm.  This  will  usually 
require  a  week.  No  post  should  be  used  until  it  is  a  month  old. 

Hence  most  molds  are  arranged  to  unclamp  so  as  to  be  easily  removed 
from  the  post,  leaving  it  lying  on  the  bottom  board.  While  there  are 
machines  for  making  posts  in  vertical  position,  it  will  generally  be  found 
more  practicable  to  make  them  horizontally,  placing  the  reinforcement  in 
the  proper  places  while  filling  the  mold. 

53 


Cost 

Under  average  conditions  the  cost  of  the  materials  used  in  a  concrete 
post  will  be  23  cents.  Scarcely  anywhere  can  cedar,  white  oak,  chestnut, 
or  locust  posts  compete  as  to  price,  and  when  we  consider  the  greater  life 
of  a  concrete  post  due  to  immunity  from  fire,  insepts,  and  rot,  we  can  easily 
understand  its  marvelous  popularity. 

Corner  Posts,  etc. 

Corner  posts  are  larger  than  line  posts  and  require  additional  reinforce- 
ment. Eight  by  eight  inches  without  taper,  reinforced  by  four  TVinch 
steel  rods  or  other  reinforcement  of  equal  cross-section,  makes  a  substantial 
corner  post. 

Braces  may  be  made  in  home-made  molds  5  inches  square  and  10  feet 
long,  with  proper  bevel  at  the  end  and  four  %-inch  steel  rods  for  reinforce- 
ment. Lugs  may  be  cast  on  the  corner  posts  to  engage  the  braces,  or  a 
mortise  made  in  the  face  of  the  corner  post. 

On  account  of  the  small  number  required,  gate  posts  will  warrant  addi- 
tional cost  and  should  be  plain  but  massive,  thereby  materially  adding  to 
the  appearance  of  the  fence  and  indirectly  enhancing  the  value  of  the  farm. 


VIII.  Concrete  Walks  and  Curbs 

I.  Concrete  Sidewalks 
Economy  and  Durability 

When  compared  with  any  other  material  suitable  for  sidewalks,  the 
low  cost  and  permanence  of  concrete  have  resulted  in  its  almost  uni- 
versal adoption,  in  enterprising  communities.  But  it  is  essential  that  no 
one  engage  in  the  construction  of  anything  so  important  as  sidewalks  are 
to  the  welfare  of  the  community  without  thoroughly  investigating  the 
principles  upon  which  success  depends,  and  becoming  entirely  familiar 
with  the  best  modern  practice. 

One-  and  Two-course  Walks 

In  the  early  days  of  concrete  walk  construction  it  was  the  universal 
practice  to  use  a  base  of  lean  concrete  over  which  was  spread  a  mortar 
top  varying  from  %  inch  to  1  inch,  made  of  cement  and  sand  or  cement  and 
stone  screenings.  This  top  or  wearing  surface  was  usually  troweled  to  a 
"glassy"  surface,  under  the  belief  that  a  very  smooth  surface  made  a 
stronger  appeal  to  the  public  eye.  Very  serious  objections  to  this  practice 

54 


have  arisen.  Not  only  has  a  surface  so  finished  been  found  slippery  and 
dangerous  to  pedestrians,  especially  in  winter  weather,  but  in  cases  of 
careless  construction  the  imperfect  bond  between  the  top  and  base  has 
resulted  in  the  two  separating,  causing  ultimate  failure  of  the  walk. 

In  the  one-course  construction  recently  introduced  a  single  mass  or 
thickness  of  well-made  concrete  is  used,  and  the  surface,  instead  of  being 
troweled,  is  finished  with  a  wooden  or  cork  float,  resulting  in  an  even  but 
not  smooth  tread,  which  overcomes  the  objection  of  the  slippery  troweled 
surface.  Using  one  mixture  throughout  the  walk,  all  of  which  is  placed 
and  tamped  at  one  operation,  does  away  with  any  possible  line  of  cleavage 
or  separation,  consequently  insuring  absolute  permanency,  the  entire  slab 
being  a  homogeneous  unit. 

Some  sidewalk  contractors  still  feel  that  a  saving  in  cost  may  be  effected 
by  using  a  leaner  concrete  for  the  base.  To  correct  this  impression,  data 
have  been  collected  showing  the  comparative  cost  of  one-  and  two-course 
walks. 

In  one-course  walks  4^  inches  of  the  richer  mixture  is  undoubtedly 
equal  in  strength  to  5  inches  of  the  two-course  walk,  using  a  leaner  mixture 
for  the  base. 

The  following  table  gives  the  cost  of  materials  used  in  the  construction 
of  100  square  feet  of  sidewalk,  and  is  based  upon  the  following  prices: 

Portland  cement,  $1.50  per  barrel  net;  sand,  $1.25  per  cubic  yard; 
gravel,  $1.50  per  cubic  yard.  (It  should  be  understood  that  on  account  of 
freight  on  long  hauls  the  cement  will  sometimes  cost  twice  as  much  as 
shown.) 


MIXTURE 

BBLS. 
CEMENT 

Cu.  YDS. 
SAND 

Cu.  YDS. 
GRAVEL 

TOTAL  COST, 
MATERIALS 

5-inch  two-course  .  .  . 
4J^-inch  one-course  .  . 

/  1:2^:5  base) 
11:1)4  top      / 
1:2:3 

2.52 
2.42 

.80 

.73 

1.21 
1.08 

$6.79 
6.16 

Materials 

In  the  construction  of  concrete  sidewalk,  as  in  all  other  concrete  con- 
struction, a  standard  brand  of  Portland  cement  should  be  used. 

Fine  aggregate  should  consist  of  sand,  crushed  stone  (free  from  dust), 
or  gravel  screenings,  graded  from  fine  to  coarse  and  passing  a  screen  of 
34-inch  mesh.  It  should  be  clean  and  free  from  foreign  matter.  On 
account  of  resistance  to  abrasion  granite  screenings  have  been  used  ex- 
tensively for  the  wearing  surface  where  there  is  considerable  traffic. 

Coarse  aggregate  should  consist  of  clean,  well-graded  gravel  or  broken 

55 


stone,  varying  in  size  from  M  inch  to  IK  inches.  Bank-run  gravel  should 
never  be  used  without  screening  and  remixing  in  the  proper  proportions; 
it  usually  contains  an  excess  of  fine  material,  rendering  proportions  un- 
certain and  indefinite.  If  the  gravel  contains  loam,  clay,  or  other  for- 
eign matter,  it  should  be  washed  before  being  used. 

Proportions 

In  the  construction  of  one-course  walk  the  materials  should  be  mixed 
in  the  proportions  of  1  sack  of  Portland  cement,  2  cubic  feet  of  fine  aggre- 
gate, and  3  cubic  feet  of  coarse  aggregate. 

In  the  construction  of  two-course  walk  the  concrete  for  the  base  should 
be  mixed  in  the  proportions  of  1  sack  of  Portland  cement,  2^  cubic  feet 
of  fine  aggregate,  and  5  cubic  feet  of  coarse  aggregate. 

In  two-course  work  the  top,  or  wearing,  surface  should  consist  of  mortar 
mixed  in  the  proportions  of  1  sack  of  Portland  cement  to  not  more  than  2 
cubic  feet  of  fine  aggregate. 

Mixing 

The  importance  of  thoroughly  mixing  the  materials  in  the  construc- 
tion of  concrete  sidewalk  cannot  be  too  strongly  emphasized.  Whenever 
possible,  a  power  batch-mixer  should  be  installed.  On  a  contract  of  any 
considerable  size  power  mixing  will  be  cheaper  than  hand  mixing,  and  every 
contractor  has  found  the  work  of  mixing  by  hand  so  laborious  that  the 
fatigue  of  the  men  has  a  marked  effect  upon  the  quality  of  the  concrete. 

Concrete  Mixers 

Batch  mixers  consist  mainly  of  a  rotating  drum  driven  by  steam,  gaso- 
lene engine,  or  electric  motor.  Both  the  shape  of  the  drum  and  the  use  of 
inside  deflectors  are  relied  upon  to  secure  thorough  mixing.  The  order  in 
which  the  material  is  discharged  from  a  batch  mixer  is  independent  of  the 
order  in  which  the  materials  are  placed  in  the  mixer.  Hence  all  materials 
required  for  one  batch  are  dumped  into  the  mixer  at  one  time,  no  attention 
being  given  to  the  order  in  which  they  are  introduced.  After  the  drum 
has  made  a  few  revolutions  water  in  measured  quantity  should  be  added, 
and  the  mixing  continued  for  a  specified  time  or  definite  number  of  revolu- 
tions. A  mixer  must  always  be  run  slowly,  to  secure  the  best  results. 

Continuous  mixers  consist  mainly  of  a  number  of  hoppers  for  the  several 
materials,  placed  over  one  end  of  a  semi-circular  trough  containing  blades 
or  vanes  fixed  to  a  rotating  shaft.  Motive  power  is  generally  supplied  by 
a  gasolene  engine  or  an  electric  motor.  Dry  materials  are  fed  automatic- 
ally from  the  hoppers  into  the  trough,  where  water  is  added  and  the  mass 
carried  along  by  the  blades  to  the  discharge  end. 

56 


Mixers  of  the  batch  type  give  better  results,  because  the  mixing  is 
under  the  operator's  control,  and  may  be  continued  until  the  materials 
of  each  batch  are  perfectly  mixed.  Moreover,  the  measuring  of  materials 
can  be  regulated  positively,  whereas  with  continuous  mixers  variation  in 
the  amount  of  moisture  in  the  sand,  fluffiness  of  the  cement,  or  arching  of 
material  in  the  hoppers  will  vary  the  relative  proportions  of  the  different 
materials  in  the  mixture. 

Consistence 

Concrete  used  for  one-course  walk  should  be  wetter  than  a  mixture 
used  for  the  base  of  two-course  walk,  sufficient  water  being  used  to  make 
a  quaky  consistence.  Enough  water  should  be  added  so  that  when  the 
concrete  is  placed  and  lightly  tamped  the  mortar  will  flush  to  the  surface 
and  make  finishing  easy. 

Mortar  for  the  wearing  surface  of  two-course  walk  should  be  mixed 
to  such  consistence  that  it  will  spread  under  a  straight-edge  resting  on  the 
forms,  but  should  not  be  wet  enough  to  cause  excess  water  to  stand  on  the 
surface  after  finishing  with  a  wooden  float.  If  surplus  water  appears  on 
the  top  of  the  mortar,  after  floating,  it  must  be  taken  up  with  a  sponge  or 
'mop.  The  practice  of  throwing  dry  cement  on  a  finished  surface  to  take 
up  surplus  water  should  be  condemned. 

Subgrade 

"Subgrade"  is  the  term  applied  to  the  surface  of  natural  soil  as  pre- 
pared to  receive  the  subbase,  or  to  receive  the  sidewalk  directly  where  a 
subbase  is  unnecessary.  The  subgrade  should  not  only  be  level,  but  should 
be  practically  uniform  in  density.  If  there  are  any  holes  or  soft  spots  in 
the  ground,  they  should  be  filled,  and  the  filling  be  tamped.  In  the  case  of 
a  fill  the  earth  should  be  tamped  in  layers  not  exceeding  6  inches  in  thick- 
ness, which  should  extend  at,  least  one  foot  on  each  side  of  the  walk,  the 
sides  having  a  slope  of  1  to  !}/£.  The  subgrade  should  have  a  slope  (toward 
the  curb  on  city  streets)  of  ^  inch  to  the  foot,  to  allow  for  drainage,  and 
should  be  11  inches  below  the  finished  surface  of  the  walk,  except  when  no 
subbase  is  required,  in  which  case  the  subgrade  should  be  5  inches  (or  4J/2 
inches)  below  the  finished  surface  of  the  walk,  depending  on  whether  the 
walk  is  of  two-course  or  one-course  construction. 

Subbase 

The  subbase  is  the  foundation  for  the  walk;  it  is  laid  on  the  subgrade, 
and  is  immediately  underneath  the  concrete  base.  The  subbase  should 
consist  of  broken  stone  from  which  the  fine  particles  have  been  removed 
by  screening,  coarse  gravel,  cinders,  or  blast  furnace  slag,  the  idea  being  to 

57 


secure  a  porous  material  through  which  water  will  readily  drain.  The 
subbase  should  be  6  inches  in  thickness,  laid  directly  on  the  subgrade  and 
thoroughly  tamped.  On  fills,  the  subbase  should  be  the  full  width  of  the 
fill,  and  the  sides  should  have  the  same  slope  as  the  sides  of  the  fill,  namely, 
1  to  1^/2.  Wherever  the  climate  is  such  that  freezing  occurs  during  the 
winter,  the  subbase  is  an  essential  part  of  concrete  walk  construction. 
Only  where  there  is  no  danger  of  frost,  or  where  there  is  perfect  drainage, 
can  the  subbase  be  safely  discarded  and  the  concrete  base  be  laid  directly 
upon  the  subgrade. 

Forms 

Forms  may  be  made  from  2-inch  lumber,  the  width  being  determined 
by  the  height  of  the  walk,  usually  4J^  inches  in  the  case  of  one-course 
walk  and  5  inches  in  the  case  of  two-course  walk.  Thirty-six  square  feet 
should  be  adopted  as  the  maximum  area  of  a  single  slab,  and  6  feet  as  the 
greatest  dimension  permissible.  Places  where  the  cross-pieces  join  the 
side  forms  should,  in  two-course  construction,  be  very  plainly  marked,  so 
that  when  the  wearing  surface  is  laid,  the  final  grooving  may  coincide 
with  the  joint  in  the  base.  Forms  must  be  kept  well  cleaned  and  must  not 
be  used  on  a  new  job  if  concrete  from  the  last  job  is  sticking  to  their  face. 
Several  well-designed  steel  forms  are  now  manufactured,  which  may  be 
advantageously  used  whenever  the  area  of  walk  to  be  constructed  will 
justify  the  initial  expenditure.  Construction  will  be  more  uniform  if  such 
forms  are  used,  and  in  the  long  run  they  will  more  than  pay  for  themselves. 

Placing 

In  constructing  one-course  walk  the  concrete  should  be  placed  and 
tamped  to  a  thickness  of  4^  inches.  Steel  tampers  are  used,  varying 
from  6  by  6  to  10  by  10  inches.  For  the  finishing  of  one-course  walk  a 
steel  tamper  with  a  face  8  inches  square  is  preferable.  A  commercial 
type  has  pyramidal  projections,  which  force  the  coarse  aggregate  below 
the  surface,  leaving  the  finer  particles  at  the  top,  ready  for  finishing,  with 
a  wooden  float;  a  steel  trowel  should  never  be  used  for  finishing  any  walk. 

In  constructing  two-course  walk  the  concrete  should  be  placed  and 
tamped  to  a  depth  of  4}^  inches,  allowing  %  inch  for  the  wearing  surface, 
which  will  be  mixed  separately,  and  must  be  placed  as  rapidly  as  possible 
after  the  placing  of  the  base.  If  any  considerable  time  elapses  between 
placing  the  base  and  laying  the  wearing  surface  thereon,  the  bond  between 
the  two  will  be  in  danger. 

Finishing  of  the  wearing  surface  or  face  may  be  done  in  several  ways, 
and  while  the  use  of  a  wooden  float  is  always  preferable,  there  are  those 
who  still  wish  the  surface  troweled.  If  troweling  is  done,  it  should  be  as 

58 


lightly  as  possible, 'in  order  to  prevent  the  formation  of  fine  cracks  and 
checks  as  well  as  a  glassy  surface. 

The  wearing  surface  should  be  cut  through  with  a  trowel  directly  over 
the  joints  in  the  base,  and  the  groover  run  over  the  surface  along  the  joint. 
Sides  should  be  finished  with  an  edger  having  a  ^-inch  radius. 

If  the  laying  of  slabs  is  continuous,  the  cross-pieces  should  be  removed 
when  a  slab  has  been  completed,  and  the  material  for  the  next  slab  placed 
immediately.  In  order  to  insure  perfect  joints  between  slabs  it  is  becom- 
ing quite  common  to  construct  slabs  alternately.  In  this  way  the  cross- 
pieces  are  allowed  to  remain  until  the  cement  has  partially  hardened  before 
being  removed  and  the  material  for  adjoining  slabs  placed.  In  this 
manner  the  slabs  form  distinct  units,  and  are  not  so  likely  to  break  in  case 
of  any  future  settlement  in  the  foundation.  The  same  result  may  be 
attained  by  using  metal  cross-pieces  remaining  in  place  until  the  concrete 
has  partially  hardened. 

Coloring 

If  it  is  desired  to  vary  the  natural  color,  the  use  of  lamp  black,  iron 
oxide,  or  any  mineral  coloring-matter  is  allowable,  provided  it  is  thoroughly 
mixed  with  the  dry  sand  in  quantities  not  exceeding  5  per  cent,  of  the  weight 
of  the  cement.  Accuracy  in  measurement  and  thorough  mixing  are  ex- 
tremely necessary  if  uniform  color  is  to  be  expected. 

Protection 

As  soon  as  the  concrete  has  hardened  sufficiently  to  prevent  the  sur- 
face from  being  pitted  it  should  be  sprinkled  with  clean  water  and  kept  wet 
for  at  least  four  days  and  not  be  exposed  to  traffic  until  thoroughly 
hardened. 

Freezing 

Under  ordinary  circumstances  the  construction  of  concrete  walk  dur- 
ing freezing  weather  is  not  advocated.  If  circumstances  make  it  imperative 
to  proceed  with  the  work  at  such  time,  the  requirements  on  all  concrete 
work,  such  as  heating  the  water  and  aggregates,  must  be  observed,  and 
precautions  must  be  taken  to  protect  the  work  from  freezing  for  at  least 
five  days  after  placing  the  concrete.  It  is  essential  that  both  the  sub- 
grade  and  the  subbase  should  be  free  from  frost  when  the  walk  is  laid. 

Expansion  Joints 

Expansion  joints  should  be  50  feet  apart  and  ^  inch  wide.  They 
should  be  filled  with  tar,  prepared  felt,  or  some  other  material  which  will 
retain  elasticity  under  changing  atmospheric  conditions. 

59 


Precautions 

Walks  should  be  grooved  where  crossed  by  driveways,  and  if  a  two- 
course  walk,  the  wearing  surface  should  be  two  inches  in  thickness  at  the 
driveway  crossing. 

Where  a  new  walk  joins  an  old  one  and  either  the  grade  has  been 
changed  or  the  old  walk  was  not  properly  laid  to  grade,  laying  an  entire 
slab  at  the  grade  necessary  to  joint  the  two  walks  will  avoid  the  unpleasant 
and  dangerous  beveling  that  is  sometimes  seen. 

In  laying  a  walk  around  trees  6  inches  clearance  should  be  allowed  to 
provide  for  future  growth.  The  character  of  the  trees  should  be  investi- 
gated, as  trees  having  lateral  roots  on  or  near  the  surface  of  the  earth  are 
almost  certain  to  cause  trouble  at  some  time. 


II.  Concrete  Curb  and  Gutter  Combined 

Combined  curb  and  gutter  is  recommended  only  for  streets  which  are 
not  to  be  improved  by  permanent  pavement.  Where  a  street  is  merely 
graded  or  surfaced  with  disintegrated  granite  or  some  similar  material,  it 
is  necessary  to  construct  concrete  gutter  in  connection  with  the  curb. 

Similarity  to  Sidewalk  Construction 

Concrete  curb  and  gutter  is  closely  associated  with  concrete  sidewalk 
construction,  not  only  on  account  of  its  position  when  placed,  but  because 
the  materials  and  method  of  using  them  are  much  the  same. 

WThat  has  already  been  said  in  this  lesson  in  regard  to  cement  selection 
of  fine  aggregate  and  selection  of  coarse  aggregate  for  concrete  sidewalk 
applies  equally  to  concrete  curb  and  gutter. 

Materials  must  be  mixed  with  the  utmost  thoroughness,  and  a  batch- 
mixer  should  be  used  whenever  possible.  If  the  mixing  must  be  done  by 
hand,  it  should  be  upon  a  water-tight  platform,  according  to  the  best 
methods,  which  involve  spreading  the  sand,  then  the  cement,  mixing  them 
until  of  uniform  color,  incorporating  the  coarse  aggregate,  adding  water, 
and  turning  the  entire  mass  at  least  three  times,  or  until  of  uniform  con- 
sistence. 

As  in  the  case  of  concrete  sidewalk,  concrete  curb  and  gutter  must  be 
carefully  protected  after  placing.,  and  must  be  kept  thoroughly  wet  for  the 
first  four  days. 

Precautions  must  be  taken,  when  necessary,  to  protect  from  frost  for  a 
period  of  five  days,  and  both  the  subbase  and  subgrade  must  be  entirely 
free  from  frost  at  the  time  of  placing  the  concrete. 

The  concrete  should  be  mixed  to  a  quaky  consistence,  so  that  water 

60 


will  flush  to  the  surface  under  slight  tamping.  Mortar  for  the  wearing 
course  must  be  of  such  consistence  that  it  will  not  require  tamping,  but 
can  be  easily  spread  into  position. 

All  the  above  requirements  are  substantially  the  same  as  for  the  con- 
struction of  concrete  sidewalk. 

Subgrade 

The  subgrade  must  be  level,  firm,  and  free  from  soft  places.  If  filled, 
the  earth  must  be  tamped  in  layers  not  exceeding  6  inches  in  thickness. 
Whether  a  fill  or  an  excavation,  the  surface  must  be  finished  11  inches  below 
the  established  grade  of  the  gutter. 

Subbase 

Upon  the  subgrade  must  be  laid  the  subbase  consisting  of  suitable 
porous  material,  such  as  slag,  cinders,  large  gravel,  or  broken  stone,  from 
which  the  finer  pieces  have  been  screened,  and  this  material  must  be 
thoroughly  compacted  and  rolled  to  a  thickness  of  5  inches,  so  that  its 
surface  will  be  6  inches  below  the  established  grade  of  the  gutter.  The 
above  measurements  are  given  with  reference  to  the  grade  of  the  gutter, 
which  is  itself  6  inches  below  the  grade  of  the  curb. 

Construction 

In  combined  curb  and  gutter,  the  depth  of  the  back  will  be  12  inches, 
the  depth  of  the  face  6  inches,  the  breadth  of  the  gutter  from  16  inches  to  24 
inches,  and  the  sections  from  5  feet  to  8  feet  in  length,  with  ^o  inch  expan- 
sion joints  occurring  every  150  feet.  Expansion  joints  should  be  filled 
with  tar,  prepared  felt,  or  other  suitable  material  which  will  retain  elasticity 
under  changes  of  temperature,  and  the  abutting  corners  should  be  rounded. 
The  necessity  for  liberal  expansion  joints  between  the  curb  and  the  side- 
walk deserves  especial  emphasis,  as  cases  of  concrete  curb  and  gutter  failing 
through  lateral  pressure  from  expanding  sidewalk  are  numerous  and  un- 
necessary. 

Two-inch  lumber  may  be  used  for  forms,  except  at  street  corners,  where 
the  radius  should  not  be  less  than  6  feet,  and  may  be  provided  for  by  sub- 
stituting metal  strips  in  place  of  the  usual  lumber.  Metal  cross-pieces 
must  be  provided  between  sections,  and  their  position  distinctly  marked 
upon  the  front  and  back  pieces  of  the  forms  in  order  that  the  wearing 
course,  when  applied,  may  be  cut  through  and  grooved  exactly  above  the 
joint  in  the  concrete  base.  The  slope  of  the  gutter  may  be  regulated  and 
maintained  by  using  an  ordinary  wooden  level  with  a  nail  driven  in  the 
bottom  at  one  end,  and  extending  out  a  distance  equal  to  the  required  pitch. 

61 


The  street  side  of  the  gutter  will  be  raised  at  the  approach  to  street-cross- 
ings, so  that  it  may  conform  to  the  grade  at  the  sidewalk  crossing. 

A  number  of  very  satisfactory  varieties  of  forms  are  now  manufactured 
from  sheet  steel.  They  are  not  only  more  durable  than  wood,  but  are 
held  in  place  by  templets,  which  do  away  with  the  necessity  of  cross-pieces 
and  eliminate  the  clamps  required  in  connection  with  wooden  forms  to 
hold  in  place  the  board  forming  the  face  of  the  curb.  This  is  a  distinct 
advantage,  affording  considerable  saving  of  time,  while  the  templets 
themselves  satisfactorily  divide  the  finished  curb  and  gutter  into  sections, 
which  is  a  point  of  considerable  importance  in  view  of  the  disaster  which 
might  otherwise  follow  a  foundation  failure. 

Placing 

In  the  construction  of  two-course  concrete  curb  and  gutter  the  concrete 
should  be  mixed  in  proportions  of  1  sack  of  Portland  cement,  2^  cubic 
feet  of  fine  aggregate,  and  5  cubic  feet  of  coarse  aggregate.  Mortar  for 
the  wearing  course  should  be  mixed  in  the  proportion  of  1  sack  of  Portland 
cement  to  2  cubic  feet  of  sand  or  other  suitable  fine  aggregate. 

Concrete  mixed  in  the  proportions  above  specified  should  be  deposited 
in  the  forms  and  thoroughly  rammed  to  place,  allowing  %-inch  for  wearing 
surface,  the  latter  to  be  applied  as  quickly  as  possible  in  order  to  secure 
an  effective  bond  between  the  base  and  the  wearing  surface. 

Three  different  methods  of  applying  the  wearing  surface  have  been 
used.  The  first  consists  in  applying  the  mortar  to  the  top  of  the  gutter 
and  the  top  of  the  curb,  and  as  soon  as  these  have  been  finished,  removing 
the  form  from  the  face  of  the  curb  and  troweling  the  mortar  down  the 
vertical  face.  This  method  is  unsatisfactory  for  several  reasons.  It  often 
results  in  the  face  of  the  curb  being  but  thinly  covered,  it  necessitates  the 
use  of  too  dry  a  mixture  on  the  vertical  face,  and  results  in  excessive  trowel- 
ing which  develops  hair  cracks  and  checking  on  the  wearing  surface.  The 
second  method  consists  in  plastering  on  the  inside  of  that  portion  of  the 
form  making  the  vertical  face  or  street  side  of  the  gutter  %  inch  of  plastic 
mortar,  the  form  being  filled  with  concrete  at  the  same  time,  so  that  the 
introduction  of  the  mortar  and  concrete  is  practically  simultaneous. 
When  the  form  lacks  ^  inch  of  being  filled,  the  top  is  then  filled  with 
mortar.  After  removing  the  forms,  the  only  work  remaining  is  the  fin- 
ishing of  corners.  A  1^-inch  radius  is  given  to  the  curb  on  the  street  side 
and  the  intersection  of  the  curb  and  gutter;  all  other  edges  are  rounded  to 
a  ^g-inch  radius  unless  protected  by  metal.  The  third  method  differs  from 
the  second  only  in  slipping  a  1-inch  board,  6  inches  wide  and  surfaced  on 
one  side,  inside  of  that  portion  of  the  form  making  the  face  of  the  curb. 
When  the  form  has  been  filled  with  concrete  this  board  is  withdrawn,  and 

62 


the  space  left  by  its  withdrawal  is  then  filled  with  plastic  mortar.  The- 
second  method  will  usually  secure  a  better  bond  between  the  base  and  the 
wearing  surface. 

Excessive  troweling  is  too  often  practised  in  finishing  the  wearing  sur- 
face of  concrete  curb  and  gutter.  A  large  part  of  the  finishing  may  be 
better  accomplished  by  the  use  of  a  stiff  fiber  brush  which  will  give  a 
more  durable  surface,  less  likely  to  develop  hair-cracks  and  checking. 

One-course  Work 

Concrete  curb  and  gutter  has  not  yet  been  so  extensively  constructed 
after  the  one-course  method  as  has  one-course  sidewalk,  but  one-course 
construction  is  likely  ultimately  to  supersede,  to  a  great  extent,  two- 
course,  on  account  of  greater  durability,  more  permanent  wearing  surface, 
and  the  saving  in  time  and  labor.  In  one-course  work  the  concrete  should 
be  prepared  in  proportions  of  1  sack  of  Portland  cement,  2  cubic  feet  of 
fine  aggregate,  and  3  cubic  feet  of  coarse  aggregate,  using  a  tamper  with 
pyramidal  or  similarly  formed  projections  that  will  drive  the  coarse  aggre- 
gate below  the  surface  and  leave  the  mortar  on  top  for  finishing  with  a 
wooden  float. 


III.  Concrete  Curb 

In  all  cases  where  there  is  a  probability  of  permanent  road  improvement 
concrete  curb  should  be  constructed  without  gutter,  as  the  gutter  will  be 
provided  by  the  slope  of  pavement  adjoining  the  curb.  By  this  method 
of  construction  the  longitudinal  joint  separating  the  pavement  from  the 
curb  is  at  the  extreme  edge  of  the  pavement,  and  the  objectionable  longi- 
tudinal joint  between  pavement  and  gutter  is  eliminated. 

Building  concrete  curb  without  gutter  is  very  simple.  There  is  no 
occasion  for  making  the  wearing  surface  richer  than  the  main  body  of 
concrete;  consequently  the  entire  curb  should  be  of  1:2:3  concrete  finished 
with  a  wooden  float,  as  recommended  for  one-course  curb  and  gutter  work. 

For  constructing  concrete  curb  without  gutter,  the  subgrade  should  be 
finished  30  inches  below  the  established  grade  of  the  work.  The  subbase 
should  occupy  6  inches,  making  its  surface  24  inches  below  the  established 
grade  of  the  curb.  Concrete  curb  should  be  12  inches  wide  at  the  base, 
6  inches  wide  at  the  top,  24  inches  high,  and  have  a  batter  on  the  street 
side  of  1  to  4. 


63 


Laboratory  Guide  for  an  Elementary  Course  in 

Concrete  Work 

To  AID  instructors  in  planning  and  conducting  an  elementary  course  in 
concrete  construction,  the  Association  of  American  Portland  Cement 
Manufacturers  has  prepared  the  outlines  and  suggested  exercises  which 
follow.  As  the  course  is  now  being  given  in  a  number  of  schools,  four  hours 
per  week  for  thirty-six  weeks  are  devoted  to  this  work;  but  by  condensing 
the  outline  and  omitting  some  of  the  laboratory  exercises,  the  time  may  be 
decreased  about  one-half,  without  entirely  omitting  any  of  the  essentials 
of  elementary  theory  and  practice.  An  effort  has  been  made  to  present 
the  work  in  a  logical  progressive  order,  but  the  arrangement  may  be  al- 
tered as  seems  necessary  to  meet  particular  requirements. 

Scope  of  the  Course 

This  course  may  be  considered  as  divisible  into  four  parts : 

1.  Class-room  work,  consisting  of  lectures  and  recitations. 

2.  Sketching  and  drawing. 

3.  Building  forms  and  equipment. 

4.  Preparing,  placing,  curing,  and  testing  the  concrete. 

What  proportion  of  the  total  time  available  is  to  be  given  to  each  of 
these  divisions  will  depend  largely  upon  the  features  that  the  instructor 
desires  to  emphasize  most;  but  in  cases  where  the  time  available  will  not 
permit  due  attention  to  all  parts  of  the  work,  sketching,  drawing,  and  form 
building  should  be  sacrificed  rather  than  class-room  work  and  practice  in 
concreting. 

In  concreting  courses  of  this  kind  now  being  taught  elsewhere,  about 
one-half  of  the  total  time  is  devoted  to  lectures, ' '  shop  talks, ' '  and  recitations, 
these  preceding  the  other  work  under  each  division  head  in  the  outline. 

General  Notes 

Many  instructors  have  found  that  the  best  results  can  be  obtained  by 
separating  the  class  into  groups  of  from  three  to  six  pupils.  Articles  to  be 
made  are  classified  in  groups,  and  each  student  is  given  a  choice  of  several 
exercises  in  each  group.  When  a  group  of  students  has  forms  ready  to  fill 
and  has  calculated  quantities  of  materials  needed,  one  large  batch  of  con- 
crete is  mixed  by  the  group,  from  which  all  the  forms  are  filled.  This  af- 
fords the  students  better  practice  than  if  each  one  were  allowed  to  mix  up 
only  enough  for  some  small  flower-box  or  other  exercise,  although  each  stu- 
dent may  mix  his  own  batch  when  making  test  specimens. 

Greater  interest  is  likely  to  result  if  it  is  possible  to  make  short  trips  of 
inspection  to  some  practical  concrete  work  in  process.  Saturday  afternoons 

64 


may  well  be  taken  up  in  this  way,  and  the  student  given  an  opportunity  to 
observe  concrete  work  as  it  is  carried  on  in  actual  practice,  and  have  his 
attention  called  to  examples  of  good  and  poor  work.  Local  contractors  and 
their  foremen  are  generally  quite  willing  to  answer  questions  and  explain 
their  methods. 

Most  of  the  following  exercises  can  be  used  in  farm  mechanics  work, 
as  well  as  in  manual  training  courses.  The  primary  object  is  to  teach  the 
elementary  principles  of  concrete  construction,  and  this  must  be  borne  in 
mind  in  the  selection  of  exercises.  At  the  start,  simple  and  useful  articles, 
instead  of  more  elaborate  pieces,  are  to  be  preferred,  but  as  the  course  pro- 
ceeds more  complicated  work  may  be  undertaken.  Artistic  possibilities 
are  limited  only  by  the  ingenuity  of  the  students  in  designing  and  construct- 
ing the  necessary  forms. 

Suggested  Laboratory  Instructions 
Report  on  Laboratory  Exercises 

A  written  report  should  be  required  from  each  student  for  each  exercise 
assigned.  Students  should  provide  themselves  with  note-books  in  which 
all  data  should  be  recorded.  No  credit  should  be  given  for  reports  not 
based  upon  the  original  data  taken  in  the  laboratory  and  entered  in  the 
laboratory  note-book.  Reports  should  be  written  in  ink. 

Reports  should  be  turned  in  not  later  than  one  week  after  the  exercise 
has  been  completed,  and  should  be  returned  for  corrections  one  week  later. 
The  form  of  report  given  below  is  suggested: 

Report  Sheet 

Name Date 

Title 

Object 

Hours'  time  spent  on  exercise 

Apparatus  used 


Method  of  performing  experiments. 


Conclusions 

5  65 


The  drawings,  descriptions,  tables,  and  all  calculations,  together  with 
answers  to  questions,  should  accompany  the  report  sheet. 

Suggested  Exercises  for  Elementary  Course  in  Concrete 

Construction 

To  be  designed  by  the  student: 

1.  Building  blocks. 

2.  Horse  block. 

3.  Duck  pond. 

4.  Bird  bath. 

5.  Concrete  foundation. 

6.  Hotbed  frame. 

7.  Small  trough. 

8.  Rain  barrel. 

9.  Stock  tank. 

10.  Fish  aquarium. 

11.  Outdoor  swimming  pool. 

12.  Ice-box. 

13.  Greenhouse. 

14.  Fence-posts. 

15.  Hitching-post. 

16.  Sun-dial. 

17.  Flower-box. 

18.  Garden-seat. 

19.  Lawn  pedestal  for  flower  urn. 

20.  Sidewalk  tiles. 

Equipment  Required 

In  a  number  of  schools  a  portion  of  the  basement  has  been  used  for  the 
concrete  laboratory,  but  the  room  selected  should  have  plenty  of  light  and 
a  smooth,  level  floor.  Much  of  the  work  will  naturally  be  done  in  the 
class-room  and  woodshop.  Most  of  the  following  equipment  can  be  made 
by  the  students  in  the  woodshop  classes: 

1.  A  well-made  mixing  platform.  It  will  be  cheaper  to  build  a  good  one 
at  the  outset  than  to  waste  time  and  money  in  constructing  and  using  tem- 
porary ones.  A  suitable  platform  can  be  built  of  2-inch  lumber  nailed 
upon  three  4  by  4's  rounded  at  the  ends.  The  platform  should  have  a 
surface  at  least  12  by  7  feet.  The  outside  4  by  4's  project  6  inches  at  both 
ends  of  the  platform,  and  are  bored  for  clevis  irons  so  that  the  platform 
may  readily  be  dragged  about. 

66 


BILL  OF  LUMBER  FOR  MIXING  PLATFORM 

12  pieces,  2  by  12  inch  by  7  feet,  dressed  on  one  side  and  two  edges  (tongued 

and  grooved  preferred). 

2  pieces,  2  by  2  inch  by  12  feet,  dressed  on  one  side  and  two  edges. 
2  pieces,  4  by  4  inch  by  13  feet,  rough. 
1  piece,  4  by  4  inch  by  12  feet,  rough. 

2.  Three  measuring  boxes  holding  1  cubic  foot,  3-^  cubic  foot,  and  34 
cubic  foot,  respectively,  and  preferably  cubical  in  shape. 

3.  Two  wood  trowels,  4  by  8  inches,  3/£  inch  thick.     (See  Plate  No.  9.) 

4.  Tamper.     (See  Plate  No.  9.)     A  tamper  weighing  between  15  and 
20  pounds  will  be  found  suitable  for  school  purposes.     If  made  of  pine,  8 
by  8  by  12  inches  or  8  by  8  by  18  inches  is  to  be  recommended,  or  if  make 
of  oak,  8  by  8  by  8  inches,  would  weigh  approximately  20  pounds.    A 
straight  handle  made  of  1 34-inch  galvanized  pipe  or  wood  will  suffice. 

5.  Four  small   open  bins  for  storing  unscreened  sand  and  gravel, 
crushed  stone,  screened  gravel,  and  screened  sand.     (As  only  small  quanti- 
ties of  aggregates  will  probably  be  kept  on  hand  at  any  one  time,  the  bins 
need  hold  only  about  3^  cubic  yard.) 

6.  Water  barrel. 

7.  Two  or  three  water-buckets. 

8.  Wheelbarrow.     (Preferably  with  a  metal  body.) 

9.  Two  or  more  square-nosed  shovels. 

10.  Screen  for  separating  sand  and  gravel.  (The  best  type  of  screen 
has  longitudinal  wires  spaced  %  mcn  apart,  with  horizontal  wires  4  to  6 
inches  apart  to  act  as  stiffeners.  Common  ^-inch  square  mesh  will  be 
found  satisfactory,  however.) 

11.  Curing  tubs  for  exercises.  (Such  tubs  can  be  made  from  oak  oil 
barrels  sawed  in  two.)  Old  turpentine  barrels'  should  not  be  used,  the 
turpentine  preventing  the  setting  of  the  mortar  or  concrete. 


Outline  of  the  Course 

I.  Materials  and  Mixtures 

1.  Classroom  Work. 

(a)  Cement,  its  qualities  and  how  to  handle  and  store  it. 
References : 

"Lessons  in  Concrete,"  No.  1. 

Bulletin  No.  26,  "Concrete  in  the  Country,"  published  by 
the  Association  of  American  Portland  Cement  Manufac- 
turers. 

General  Notes: 

The  history  of  Portland  cement  and  the  details  of  its  manu- 
facture are  interesting  subjects  to  all,  as  is  the  matter  of 
67 


testing  Portland  cement  to  determine  its  fitness  for  our  pur- 
poses. On  the  other  hand,  it  must  be  remembered  that  a 
thorough  knowledge  of  how  to  employ  cement  for  our  various 
purposes  is  much  more  important  than  a  knowledge  of  its 
origin,  and  that  tests  on  cement,  unless  conducted  under 
standard  laboratory  conditions  and  by  competent  cement 
testers,  are  not  reliable  and  mean  but  little. 

Portland  cement  of  any  standard  brand  may  be  used  without 
question,  provided  it  has  not  been  allowed  to  become  lumpy 
in  storage.  In  opening  a  sack  it  is  well  to  run  the  hand  down 
the  inside  in  search  of  lumps.  If  there  are  particles  present 
which  will  not  crumble  under  the  pressure  of  the  fingers,  the 
cement  is  unsuited  for  general  use,  although  there  may  be  no 
serious  objection  to  using  it  in  large  foundations  or  other 
mass  work  after  lumps  have  been  screened  out. 

(b)  Sand,  gravel,  and  stone. 

References: 

"Lessons  in  Concrete,"  No.  2. 

Bulletin  No.  26,  "Concrete  in  the  Country." 

General  Notes: 

The  definitions  of  the  common  materials  used  in  the  work 
should  be  made  clear  to  the  students. 

The  term  "aggregates"  refers  to  sand,  gravel,  and  stone  for 
either  mortar  or  concrete. 

" Fine  aggregate"  is  defined  as  "sand  or  crushed  stone  that 
will  pass  a  No.  4  sieve;  that  is,  a  screen  having  four  meshes 
to  the  linear  inch." 

"Coarse  aggregate"  is  material  such  as  "gravel  and  crushed 
stone  that  is  retained  on  a  No.  4  sieve."  The  largest  par- 
ticles in  the  coarse  aggregate  should  never  be  larger  than  one- 
half  the  distance  between  the  forms. 

"Bank-run  gravel"  is  the  term  applied  to  gravel  and  sand 
just  as  it  is  taken  from  the  pit,  without  being  washed  or 
screened. 

(c)  Preparation,  screening,  washing,  proportioning,  and  mixing. 

References : 

"Lessons  in  Concrete,"  No.  3. 

Editorial,  "Engineering  Record,"  May  30,  1914. 

"The  Folly  of  Using  Bank-run  Mixtures  in  Concrete." 

General  Notes: 

Screening. — Bank-run  gravel  should  never  be  used  as  it  comes 
from  the  deposit,  but  should  be  screened  and  then  recombined 
in  the  proper  proportions.  Strong,  dense,  water-tight  con- 
crete requires  strict  attention  to  proportioning.  This  pre- 
cludes the  possibility  of  using  bank-run  material  without 
screening. 

68 


Water  Used. — Water  used  must  be  clean,  free  from  oil,  acid, 
alkali,  and  vegetable  matter. 

Proportioning. — In  proportioning  by  volume,  a  sack  of  cement 
is  considered  as  one  cubic  foot,  and  by  weight,  a  sack  of 
cement  may  be  accepted  as  94  pounds  net.  Materials 
should  always  be  carefully  measured,  never  guessed  at.  On 
large  jobs  it  is  customary  to  measure  aggregates  in  multiples 
of  "one-sack  batches."  One  cubic  foot,  the  sack  of  cement, 
is  taken  as  the  unit  of  measure.  The  aggregates  are  then  pro- 
portioned with  suitable  sizes  of  measuring  boxes  varying  in 
capacity  from  1  to  3  cubic  feet. 

(d)  Theory  of  mortar  and  concrete. 
References : 
Bulletin  No.  26,  "Concrete  in  the  Country,"  p.  11. 

General  Notes: 

The  aggregates  consisting  of  sand  and  gravel  or  broken  stone 
are  wholly  inert  until  combined  with  Portland  cement. 
Consequently,  it  is  of  prime  importance  that  every  grain  of 
sand  be  enclosed  in  a  film  of  cement  and  water  and  every 
piece  of  coarse  aggregate  be  surrounded  with  cement  mortar. 

The  following  table  will  be  found  very  useful  in  calculating 
the  quantities  of  sand  and  gravel,  or  stone,  required  for  a  one- 
bag  batch  of  mortar  or  concrete,  and  in  computing  the  vol- 
ume of  the  resulting  mortar  or  concrete. 

TABLE  NO.  1 


MIXTURES 

MATERIALS 

VOL.  IN  Cu.  FT. 

Cement 

Sand 

Gravel  or 
Stone 

Cement  in 
Sacks 

Sand 
Cu.  Ft. 

Gravel  or 
Stone 
Cu.  Ft. 

Mortar 

Concrete 

1 

1.5 

1 

1.5 

1.75 

1 

2.0 

1 

2.0 

2.1 

1 

2-.5 

1 

2.5 

2.5 

1 

3.0 

1 

3.0 

2.8 

1 

1.5 

3 

1 

1.5 

3 

3.5 

1 

2.0 

3 

1 

2.0 

3 

3.9 

1 

2.0 

4 

1 

2.0 

4 

4.5 

1 

2.5 

4 

1 

2.5 

4 

4.8 

1 

2.5 

5 

1 

2.5 

5 

5.4 

1 

3.0 

5 

1 

3.0 

5 

5.8 

69 


TABLE  NO.  2 

QUANTITIES  OF  CEMENT,  SAND,  AND  GRAVEL  OR  STONE  REQUIRED 
FOR  ONE  CUBIC  YARD  OF  COMPACT  MORTAR  OR  CONCRETE 


MIXTURES 

QUANTITIES  OF  MATERIALS 

SAND 

STONE  OR  GRAVEL 

Cement 

Sand 

Gravel  or 
Stone 

Cement  in 
Sacks 

Cu.  Ft. 

Cu.  Yd. 

Cu.  Ft. 

Cu.  Yd. 

1 

1.5 

15.5 

23.2 

0.86 

1 

2.0 

12.8 

25.6 

0.95 

1 

2.5 

11.0 

27.5 

1.02 

1 

3.0 

9.6 

28.8 

1.07 

1 

1.5 

3 

7.6 

11.4 

0.42 

22'.8 

Q.S5 

1 

2.0 

3 

7.0 

14.0 

0.52 

21.0 

0.78 

1 

2.0 

4 

6.0 

12.0 

0.44 

24.0 

0.89 

1 

2.5 

4 

5.6 

14.0 

0.52 

22.4 

0.83 

1 

2.5 

5 

5.0 

12.5 

0.46 

25.0 

0.92 

1 

3.0 

5 

4.6 

13.8 

0.51 

23.0 

0.85 

Stone  and  gravel  =  45  per  cent,  voids  (average). 

1  sack  cement  =  1  cu.  ft.;  4  sacks  =  1  bbl. 

Based  on  tables  in  "Concrete,  Plain  and  Reinforced,"  by  Taylor  &  Thompson. 

It  is  necessary  occasionally  to  mix  up  quantities  of  concrete  or  mortar 
requiring  less  than  a  sack  of  cement,  and  for  small  exercises  some  other 
unit  of  proportioning  than  the  cubic  foot  is  necessary.  A  quart  measure, 
which  holds  approximately  2%  pounds  of  cement,  or  a  peck  measure,  which 
holds  approximately  22  pounds,  will  be  found  very  convenient  for  measur- 
ing small  quantities  of  cement.  When  dumped  from  the  sack,  cement 
becomes  fluffy  and  occupies  more  space  than  when  compacted  in  a  sack; 
hence  in  measuring  cement  by  volume  it  will  be  found  necessary  to  jar  it 
down  a  few  tunes  in  the  measure  in  order  to  get  accurate  results.  Both 
coarse  and  fine  aggregate  can  be  measured  by  means  of  small  measuring 
boxes  holding  ^,  Yfr  and  1  cubic  foot  respectively;  or  by  the  quart  or  peck 
measure  referred  to  above. 


TABLE  NO.  3 


1:2 

Mixture 


1:3 

Mixture 


1  sack  cement  94 
47 
32 
23.5 
16 
12 

1  sack  cement  94 
47 
32 
23.5 
16 
8 


Ibs. 


2  cu.  ft.  sand 
1 

H 

Yz 

3 

IH 

H 


MIXING 

The  first  step  in  mixing  is  to  spread  the  sand  in  a  thin  layer  over  the 
center  of  the  mixing  platform,  then  spread  the  cement  on  top  of  the  sand 

70 


and  mix  together  dry,  continuing  the  turning  until  the  color  is  uniform  and 
without  streaks.  After  the  cement-sand  mixture  has  been  turned  at  least 
twice  it  should  be  spread  in  a  thin  layer  and  the  measuring  box  placed  upon 
it.  The  proper  amount  of  screened  gravel  should  then  be  shoveled  into  the 
box  and  the  latter  lifted  off.  Mixing  is  then  continued  until  the  gravel  is 
thoroughly  distributed  throughout  the  mass;  this  will  require  turning  the 
batch  at  least  twice.  Water  is  then  added  slowly  from  a  sprinkling  can  or 
from  a  small  stream  applied  by  a  hose,  the  mixing  continued  until  all  parts 
of  the  mass  are  the  same  in  color  and  consistence  and  wet  enough  so  that 
there  is  a  tendency  to  flatten  out  when  the  mass  is  heaped  up.  The  con- 
crete must  always  be  used  within  twenty  minutes  or  half  an  hour  after  the 
water  has  been  added. 

The  quality  of  the  concrete  depends  largely  upon  the  amount  of  water 
in  the  mixture,  a  mixture  such  as  described  giving  better  results  than  a  dry 
one;  in  fact,  a  dry  mixture  is  not  capable  of  developing  all  the  strength  of 
the  cement.  Mixtures  containing  less  water  are  frequently  used  in  making 
cement  products,  but  the  practice  is  a  bad  one  and  should  be  avoided  when- 
ever possible.  Too  much  water  is  likely  to  cause  pockets  and  imperfec- 
tions in  the  surface  of  the  work,  and  increases  shrinkage  while  hardening. 

II.  Forms  and  Molds 

1.  Class-room  Work.     (Using  slides  or  charts.) 

(a)  Materials  for  making. 

References: 

"Lessons  in  Concrete,"  No.  4. 
Bulletin  No.  26,  "Concrete  in  the  Country." 
Bulletin  No.  23,  "Concrete  Tanks." 

General  Notes: 

Forms  are  made  of  wood,  metal,  and  combinations  of  wood 
and  metal. 

(b)  Various  types. 

References: 
Same  as  those  given  under  (a). 

General  Notes: 

Various  types  of  forms. 

1.  Rectangular  forms  wholly  of  lumber. 

2.  Rectangular  forms  using  metal  fastening. 

3.  Rectangular  metal  forms. 

4.  Circular  forms  of  wood  and  sheet  metal. 

5.  Circular  forms  wholly  metal. 

6.  Miscellaneous. 

(c)  General  requirements,  care,  and  use. 

References: 
Same  as  those  given  under  (a). 

General  Notes: 

Green  lumber  will  keep  its  shape  better  in  all  rectangular 
forms  than  will  lumber  that  is  thoroughly  dry.     If  dry  lum- 
71 


72 


ber  is  used,  it  should  be  thoroughly  wet  before  the  concrete  is 
placed.  White  pine  is  considered  the  best  lumber  for  forms, 
although  spruce,  fir,  and  Norway  pine  are  often  used.  The 
face  of  the  forms  should  be  free  from  knots,  slivers,  or  other 
irregularities.  The  forms  should  be  thoroughly  cleaned  each 
time  they  are  used,  so  that  no  dry  concrete  is  left  sticking  to 
the  face  of  the  forms.  The  use  of  oil  or  grease  on  the  face  of 
forms  is  recommended,  as  it  prevents  absorption  of  water 
from  concrete  and  makes  form  removal  easier.  A  mixture 
of  equal  parts  of  boiled  linseed  oil  and  kerosene  is  generally 
used  for  painting  the  forms.  Tallow  or  animal  fats  should 
not  be  used  in  painting  the  forms. 

2.  Form  Work. 

Construction,  small  wooden  molds  for  making  test  specimens. 

EXERCISE   I-A 
GANG  MOLD  FOR  TEST  SPECIMENS 

(Plates  Nos.  1  and  2) 

The  construction  of  a  small  wooden  gang  mold  for  making  test 
specimens  1^  inches  square  and  6  inches  long. 
Tools  used:   Saw,  plane,  square,  gauge  and  hammer. 

MILL  BILL  OF  MATERIAL  FOR  GANG  MOLD 

4  pieces    1  inch     x  3      inches  x  12      inches  S3S,  base  of  form. 

2  pieces  %  inch     x  1J^  inches  x  10J^  inches  S2S,  sides  of  form. 

2  pieces  %  inch     x  1  ^  inches  x   8      inches  S2S,  sides  of  form. 

4  pieces  %  pch     x  1^  inches  x    6H  inches  S4S,  movable  partitions. 

2  pieces    2  inches  x  2      inches  x  12      inches  SIS,  bottom  strips. 

2  pieces    2  inches  x  1^  inches  x    3      inches  S4S,  blocks. 

2  pieces    2  inches  x  1  ^  inches  x   5      inches  S4S,  wedges. 

(See  "General  Notes"  for  instructions  to  be  followed  in  selecting 
the  lumber  for  wooden  forms.) 

The  joints  of  the  mold  should  be  as  tight  as  possible.  This  will 
require  care  and  accuracy  in  squaring  up  the  various  pieces  before 
they  are  assembled. 

As  shown  in  the  assembled  drawing  of  the  small  gang  mold,  the  first 
thing  to  consider  would  be  the  base  of  the  form,  which  will  be  made 
either  of  four  1  by  3  inch  strips  or  two  1  by  6  inch  boards  held  in 
place  by  2-inch  strips  which  can  be  made  by  cutting  2  by  4's  length- 
wise. All  pieces  should  be  squared  up  and  surfaced  with  a  plane 
before  they  are  put  together. 

The  sides  of  the  form  can  be  made  from  strips  1  inch  thick  and  2 
inches  wide,  planed  down  to  %  inch  thick  by  V/i  inches  wide  and 
supplied  with  grooves  as  shown.  The  four  movable  partitions  are 
made  from  J^-inch  stock,  and  the  ends  are  slightly  tapered  so  that 
they  can  be  moved  in  their  respective  grooves  without  difficulty. 

Two  of  the  sides  of  the  form  are  fastened  securely  to  the  base  by 
means  of  screws,  and  the  other  two  sides  are  held  in  position  by 

73 


means  of  wedges,  which  can  easily  be  made  by  sawing  a  block  in  two 
diagonally.  After  all  the  pieces  have  been  approved  by  the  in- 
structor and  the  form  has  been  assembled,  the  faces  which  will 
come  in  contact  with  the  concrete  should  be  given  two  coats  of 
shellac. 


PLATE  2. — DETAILS  OF  FORM  FOR  TEST  SPECIMENS. 

EXERCISE  I-B  (OPTIONAL  WITH  EXERCISE  I-A) 

GANG  MOLD  FOR  TEST  SPECIMEN 

(Plate  No.  3) 

The  construction  of  this  gang  mold  is  much  more  simple  than  that 
described  in  Exercise  I-A,  and  is  preferable  for  work  with  younger 

74 


I 


boys.  As  shown  on  the  plate,  it  is  only  necessary  to  cut  and 
plane  to  size  four  strips  \Y^  by  1^  by  12  inches  and  six  blocks  !}/£ 
•  by  1^2  by  3  inches.  One  of  the  longer  strips  is  fastened  securely  in 
position  by  means  of  three  screws;  all  other  parts  of  the  mold  are 
movable  except  the  blocks  that  hold  the  wedges.  The  form  is 
designed  for  molding  three  test  specimens  of  the  same  size  as  those 
made  in  mold  I- A. 

III.  Tools  and  Equipment 

1.  Class-room  Work.     (Using  slides  and  charts.) 

(a)  List  of  the  common  tools. 

Reference: 
Bulletin  No.  26,  "Concrete  in  the  Country." 

General  Notes: 

Common  Tools. — Shovels,  pails,  tamper,  float,  edger  trowel, 
groover,  and  straightedge. 

Equipment. — Screen,  mixing  platform,  measuring  box,  wheel- 
barrow, water  barrel. 

(b)  Directions  for  constructing  floats,  wooden  trowels,  tampers, 
straightedge,  mixing  platform,  and  mixing  box. 
Reference: 

Bulletin  No.  26,  "Concrete  in  the  Country." 

General  Notes: 

For  size,  see  accompanying  drawings  and  general  directions. 
Blackboard  sketches  will  be  found  helpful  in  explaining  the 
various  tools  and  equipment. 

2.  Woodshop  Work. 

(a)  Making  a  straight  edge,  float,  wood  trowel,  measuring  box, 
tamper,  etc. 

EXERCISE  2-A 
A  DEVICE  FOR  TESTING  SMALL  CONCRETE  SPECIMENS 

(Plate  No.  4) 

This  simple  device  for  testing  small  concrete  specimens  can 
easily  be  made  by  elementary  manual  training  students.  It  is 
intended  for  use  only  in  testing  very  small  beam  specimens, 
and  even  for  such  work  much  more  satisfactory  results  can  be 
accomplished  by  the  use  of  the  home-made  testing  machine 
shown  in  Plates  Nos.  5  and  6. 

The  testing  device  consists  of  two  triangular  pieces  of  hardwood 
18  inches  long,  which  can  be  clamped  to  any  ordinary  bench  or 
table.  The  distance  apart  will,  of  course,  depend  upon  the 
length  of  the  specimens  to  be  tested.  The  specimen  should 
bear  on  the  points  at  a  distance  of  about  Yi  mch  from  each  end, 
and  the  triangular  supports  should  always  be  kept  exactly 
parallel. 

76 


1 


I 

o 


& 

« 


i 
I 


77 


The  loads  are  applied  by  adding  sand  or  water,  preferably  the 
former,  to  a  pail  suspended  by  a  hook  secured  to  a  third  tri- 
angular piece  or  saddle  resting  across  the  specimen  at  the  center. 
Sand  used  should  be  fine  and  thoroughly  dry,  so  it  will  readily 
flow  through  a  small  rubber  tube  connected  to  a  can  or  funnel, 
the  flow  being  shut  off  by  a  pinch-cock  fitted  to  the  tube. 

EXERCISE  2-B 
TESTING  MACHINE 

(Plates  Nos.  5  and  6) 

This  small,  home-made  machine  was  designed  by  the  Muncie 
(Indiana)  Normal  Institute  for  testing  specimens  either  1  by 
1  by  12  inches  or  1  by  2  by  12  inches. 

It  consists  of  levers  so  arranged  that  the  load  applied  to  the 
specimen  is  increased  ten  times  over  that  applied  to  the  ma- 
chine. It  is  equipped  with  an  automatic  cutoff,  which  prevents 
the  load  from  increasing  after  the  specimen  has  failed. 

The  frame  which  supports  the  machine  can  be  made  of  pine, 
but  the  lever  arms  should  be  of  hard  wood.  One  end  of  the 
lower  lever  supports  a  can,  which  is  located  just  below  another 
can  containing  a  supply  of  dry  sand.  At  the  other  end  of  the 
lever  a  ball  weight  is  suspended  which  can  be  adjusted  so  as  to 
balance  the  levers  just  before  the  test  is  applied. 

At  the  center  of  the  bottom  of  the  can  containing  the  sand  is  a 
J^-inch  hole  through  which  the  sand  is  allowed  to  flow  into  the 
lower  box.  An  automatic  cutoff  controlled  by  a  coiled  spring 
and  trigger  is  provided.  A  No.  9  wire  is  attached  to  the  lower 
lever  and  projects  through  a  small  opening  in  the  cutoff,  when 
the  large  hole  in  cutoff  is  directly  under  the  opening  in  the  sand 
supply  can.  This  wire  acts  as  a  trigger,  and  as  the  beam  breaks 
and  the  levers  descend,  the  wire  is  withdrawn  from  its  hole, 
releasing  the  cutoff,  which  in  turn  is  drawn  back  by  the  spring 
cutting  off  the  supply  of  sand.  The  weight  of  the  sand  in  the 
lower  can  is  determined  and  multiplied  by  ten.  This  product 
will  give  the  load  which  caused  the  specimen  to  fail. 

IV.  Walk  and  Floor  Work 

1.  Class-room  Work. 

(a)  Equipment  for  concreting. 

Reference : 

"Lessons  in  Concrete,"  No.  8. 

General  Notes: 

The  equipment  necessary  is  as  follows:  Mixing  platform, 
measuring  box,  screen,  trowels,  straightedge,  tamper,  float, 
wheelbarrow,  water  barrel,  shovels,  and  pails. 

(b)  Forms  for  flat  concrete  work,  sidewalk,  and  floor  slabs. 

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References : 

Published  lectures:    "Concrete  Walks,   Floors   and  Pave- 
ments." 
Bulletin  No.  26,  "Concrete  in  the  Country,"  pp.  28-34  inc. 

General  Notes: 

See  accompanying  Plate  No.  9  showing  one-course  and  two- 
course  walks  and  the  tools  used  in  constructing  them. 

(c)  Concrete  walk  and  floor  construction. 
Reference : 

Bulletin  No.  26,  "Concrete  in  the  Country." 
"Lessons  in  Concrete,"  No.  6.     "Concrete  Surfaces." 

2.  Form  Work. 

(a)  Construction  of  forms  for  any  of  the  following:  Sidewalks,  horse 

blocks,  curbs,  dairy  barn  floor,  monolithic  steps. 

(b)  Exercise  in  finishing  concrete  surfaces  with  wood  and  steel 
trowels,  noting  the  effect  of  excessive  troweling  with  steel 
trowel,  which  brings  the  cement  and  finer  particles  to  the  sur- 
face. 

EXERCISE  3 
FORM  FOR  CONCRETE  HORSEBLOCK 

(Plate  No.  7) 

Tools  necessary  for  constructing  the  form:  Saw,  plane,  try 
square,  gauge,  hammer. 

BILL  OF  MATERIAL 

2  pieces  2  inches  x  10  inches  x  3J^  feet 
1  piece   2  inches  x  10  inches  x  2^  feet 

1  piece   2  inches  x  10  inches  x  2  feet  8  inches 

2  pieces  2  inches  x   8  inches  x  2J^  feet 
2  pieces  2  inches  x    8  inches  x  2  feet 

Before  assembling  the  mold,  each  piece  should  be  oiled  thor- 
oughly on  both  sides  with  linseed  oil,  as  well  as  on  the  ends. 
This  will  also  prevent  any  tendency  of  the  mold  to  warp  or 
buckle. 

EXERCISE  4 

CONCRETE  PORCH  AND  STEP  CONSTRUCTION 
(Plate  No.  8) 

Tools  necessary  for  constructing  the  form:  Saw,  square,  ham- 
mer. 

The  accompanying  plate  is  self-explanatory.  The  same  pre- 
cautions should  be  taken  as  described  in  connection  with  all 
wooden  forms  as  to  selection  of  lumber  and  oiling,  so  that  the 
form  can  be  removed  easily.  It  is  possible  to  construct  the 
form  using  stock  lengths,  and  the  plank  can  again  be  used  for 
other  forms. 

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EXERCISE  5 
DETAILS  OF  CEMENT  SIDEWALK  CONSTRUCTION 

(Plate  No.  9) 

Tools  necessary  for  constructing  form:  Ax,  saw,  hammer. 
2  by  4  or  2  by  6  inch  lumber  should  be  used  for  the  forms, 
except  for  curves,  where  common  house  siding  or  other  thin 
lumber  (or  sheet  metal)  will  be  found  convenient. 
Forms  of  2  by  4  inch  stuff  should  be  used  for  a  walk  4  inches 
thick  and  2  by  5  inches  for  walks  5  inches  thick. 
3.  Concrete  Work. 

Construction  of  horseblock;  steps,  sidewalk. 

V.  Elementary  Theory  of  Reinforcement 

1.  Class-room  Work. 

(a)  Definition  of  tension  and  compression.- 

References : 

"Concrete  Plain  and  Reinforced,"  Taylor  and  Thompson. 
"Reinforced  Concrete  Construction,"  Vol.  I,  Geo.  S.  Hool. 
Published  by  McGraw  Hill  Publishing  Company,  New  York 
City. 

General  Notes: 

Concrete  is  a  material  which  is  strong  in  compression,  or 
crushing  strength,  but,  like  other  masonry,  is  weak  in  resist- 
ing tension  or  pulling  force;  therefore  it  must  be  reinforced 
with  steel,  which  is  strong  in  tension. 

(b)  Reinforcing  materials. 

References : 

"Concrete  Plain  and  Reinforced,"  Taylor  and  Thompson. 
"Reinforced  Concrete  Construction,"  Geo.  S.  Hool. 

General  Notes : 

Suitable  materials:  Steel  rods — round,  square,  and  twisted 
square,  or  of  special  section,  providing  they  have  a  sufficient 
cross-sectional  area.  Woven- wire  fabric  especially  made  for 
reinforcing,  wire  cables,  and  similar  reinforcing  are  used  ex- 
tensively. 

(c)  Most  efficient  placing  of  reinforcement  in  simple  exercises. 

General  Notes: 

Care  must  be  taken  to  place  the  steel  reinforcing  where  it  will 
do  the  most  good,  and  this,  of  course,  will  depend  upon  the 
loads,  point  of  application,  etc.  For  instance,  a  fence  post 
will  be  subject  to  different  stresses  and  strains  than  would  a 
'beam  supported  at  both  ends  with  the  load  applied  trans- 
versely; and  a  slab  in  a  vertical  position,  as  in  a  wall,  will  be 
subject  to  different  strains  than  one  in  a  horizontal  position. 
A  few  blackboard  sketches  will  help  bring  out  some  of  the 
elementary  principles  of  reinforcing. 
85 


VI.  Unit  Construction 

1.  Class-room  Work. 

(a)  Forms  for  unit  work;  reinforcing;  assembling. 

References : 
"Reinforced  Concrete  Construction,"  Vol.  II,  G.  S.  Hool. 

General  Notes: 

Often,  in  the  use  of  concrete  in  building  construction,  various 
parts  of  the  structure  are  cast  as  units,  such  as  wall  sections, 
posts  and  columns,  and  when  properly  hardened  are  as- 
sembled to  form  the  structure. 

(b)  Designing  pedestals,  sun-dials,  garden-seats,  plates  for  baseball 

diamonds,  and  other  unit  exercises  mentioned  on  page  66. 
References : 
See  plates  Nos.  10  and  11. 

General  Notes: 

There  are  many  small  articles  that  can  be  designed  by  the 
student,  such  as  foot-scraper,  door-weight,  small  tile,  home- 
plate  for  baseball  diamond,  etc. 

2.  Concrete  Work. 

(a)  Construction  of  unit  dog-house;  a  flight  of  unit  steps;  pedestal; 
garden-seat,  etc. 

EXERCISE  6 

LAWN  PEDESTAL 

(Plate  No.  10) 

There  are  three  simple  designs  shown  on  the  plate  which  lend 
themselves  nicely  to  the  material.  Forms  in  which  to  cast 
them  are  easily  constructed. 

A  1 : 2  mixture  of  cement  and  sand  is  to  be  recommended,  though 
in  some  cases  a  1:3  mixtures  might  be  found  satisfactory. 
Various  surface  effects  can  be  secured  by  using  marble  or  gran- 
ite screenings,  ground  mica,  or  combinations  of  two  or  more 
different  aggregates.  A  little  experimenting  with  available 
aggregates  will  soon  show  the  variations  in  color  possible  with- 
out resorting  to  the  use  of  artificial  coloring.  (Under  the  head- 
ing Colored  Surfaces,  additional  information  is  given  on  this 
subject.)  Scrubbing  the  surface  of  green  concrete  in  which 
variegated  colored  aggregates  have  been  used  will  result  in 
decidedly  pleasing  effects. 

FjXERCISE    7 

GARDEN  BENCH 
(Plate  No  12) 

The  plate  shows  plainly  the  construction  of  the  forms  and 
the  method  of  reinforcing. 
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The  concrete  mixture  should  be  composed  of  1  part  Portland 
cement,  2  parts  of  clean  sand,  and  2  parts  of  crushed  stone  or 
gravel,  ranging  in  size  from  ^  inch  to  }/£  inch.  If,  however, 
no  coarse  aggregate  is  used,  a  1 : 2)^  or  a  1 : 3  mixture  of  cement 
and  coarse  sand  should  be  used. 

The  slab  and  pedestals  are  cast  separately.  In  making  the 
slab,  first  fill  the  mold  uniformly  to  a  depth  of  %  inch  and  then 
lay  the  reinforcing  as  indicated  on  the  plate,  having  pre- 
viously laid  out  and  wired  together  at  intersections  the  rein- 
forcing to  correspond  with  the  dotted  lines  on  the  plate. 
On  top  of  this  place  the  remaining  2^  inches  of  concrete,  which 
should  be  wet  enough  to  require  only  slight  tamping  to  flush 
water  to  the  surface.  Following  this  the  mold  may  be  shaken 
to  cause  the  concrete  to  settle  into  corners.  This  top  surface 
will  be  the  top  of  the  finished  bench,  therefore  it  will  pay  to 
use  care  hi  finishing  it  to  as  smooth  a  surface  as  possible.  An 
edger  is  run  around  the  inside  of  the  form  to  give  a  rounded 
edge  to  the  top  of  the  slab. 
General  Notes: 

Do  not  attempt  to  remove  the  form  from  the  under  side  of 
slab  for  at  least  seven  days;  under  favorable  conditions,  the 
sides  of  the  form  can  be  removed  after  forty-eight  hours. 

VII.  Posts  and  Columns 

1.  Class-room  Work. 

(a)  Concrete  fence-post  construction.     Hitching  posts;  ornamental 

posts  and  columns. 
References : 

"Lessons  in  Concrete,"  No.  7. 

United  States  Department  of  Agriculture  Farmers'  Bulle- 
tin No.  403. 
General  Notes: 

Concrete  posts  have  been  constructed  in  a  great  variety  of 
shapes — triangular,  round,  square,  half-round,  etc.  The 
rectangular  post  commonly  used  by  farmers  is  7  feet  long, 
5  by  5  inches  at  the  bottom,  and  tapers  to  3  by  3  inches  at  the 
top.  The  length,  7  feet,  permits  placing  2^2  feet  of  the  post 
in  the  ground.  Some  prefer  an  8-foot  post. 

(b)  Corner  and  gate-posts;  size  and  bracing. 

References : 
Same  as  under  (a). 

General  Notes: 

Concrete  corner,  end,  and  gate-posts  should  be  square,  and 
the  sides  should  be  not  less  than  12  inches  wide,  and  the  post 
should  be  placed  from  3  feet  6  inches  to  4  feet  hi  the  ground. 

2.  Form  Work. 

(a)  Construction  of  forms  for  fence  line  posts  (two  designs) ;  corner 

90 


and   gate-posts    (two   designs);    hitching-posts   and   lighting 
standards. 

3.  Concrete  Work. 

(a)  Construction  of  forms  for  line  posts;  concrete  base  for  steel  and 
wooden  posts;  gate-posts;  hitching-posts;  lighting  standards. 

VIII.  Foundations  and  Piers 

1.  Class-room  Work. 

(a)  Laying  out,  and  excavating  for,  foundations. 

References : 

"  Lessons  in  Concrete,"  No.  5. 

Bulletin  No.  26,  "Concrete  in  the  Country." 

General  Notes: 

In  preparing  to  erect  any  rectangular  structure  a  base  line 
should  first  be  established,  and  from  it  the  several  corners  be 
located  by  accurate  measurement  at  right  angles,  and  then 
all  measurements  should  be  checked  back  to  the  base  line. 
The  depth  of  the  excavation  depends  upon  the  height  and 
character  of  the  building,  but  should  always  go  to  solid  earth 
and  below  frost  line. 

(b)  Forms  for  mass  construction. 

References: 

Same  as  those  under  (a). 
General  Notes: 

Forms  for  piers  and  machinery  foundations  are  constructed 

in  substantially  the  same  manner  as  are  forms  for  regular 

building  foundations. 

(c)  Repairing  old  barn  foundations. 

References : 

See  those  under  (a). 

General  Notes: 

Foundations  which  are  laid  up  in  mortar  may  disintegrate 
and  crumble,  and  this  condition  frequently  exists  in  buildings 
which  are  otherwise  in  a  fair  state  of  preservation  and  well 
worth  saving.  If  the  foundation  wall  has  not  gone  to  pieces, 
a  large  portion  of  it  can  often  be  left  in  position  and  boxed  in 
with  concrete.  If  that  part  of  the  foundation  below  ground 
is  left  in  good  condition,  it  may  be  capped  with  concrete.  It 
will,  of  course,  be  necessary  to  take  the  load  of  the  building 
off  the  foundation  temporarily,  so  that  the  crumbled  portions 
may  be  removed  and  the  new  concrete  be  given  a  chance  to 
harden.  It  may  also  be  necessary  to  replace  old  sills  and 
splice  posts. 

2.  Form  Work. 

(a)  Forms  for  foundations  (three  types). 

(b)  Forms  for  piers. 

91 


Reference : 

Bulletin  No.  26, 
3.  Concrete  Work. 


'Concrete  in  the  Country." 


(a)  Construction  of  a  section  of  foundation  wall  by  the  students. 
(Note. — The  students  will  take  a  greater  interest  in  the  work  if 
it  is  possible  to  go  out  and  do  a  little  practical  concreting  around 
the  school  grounds  or  in  the  vicinity,  such  as  building  or  repair- 
ing a  piece  of  sidewalk,  foundation  wall,  etc.) 

IX.  Ornamental  Work 

1.  Class-room  Work. 

(a)  Core  making;  shaping  of  reinforcement;  the  use  of  clay  molds; 
plaster  and  glue  molds  arid  cores;    coloring  and  finishing  sur- 
faces. 
References : 

Ralph  Davison's  book,  "Concrete  Pottery  and  Garden  Furni- 
ture." 

"Lessons  in  Concrete,"  No.  6,  The  Surface  Finish  of  Concrete. 
General  Notes: 

In  case  ornamental  work  is  to  be  attempted,  Ralph  Davi- 
son's book  should  be  obtained.  This  book  takes  up  the  sub- 
jects of  plaster  molds,  glue  molds,  and  wire  forming  in  a 
simple  manner. 

COLORED  SURFACES 

For  artistic  work  it  is  better  to  depend  upon  selection  and  combination 
of  aggregates  of  various  colors  than  upon  any  process  of  coloring  by  pig- 
ments. However,  artificial  coloring-matter,  if  used,  should  never  exceed 
8  per  cent,  of  the  weight  of  the  cement,  and  should  be  mixed  with  dry  cement 
before  water  is  added.  Nothing  but  mineral  coloring-matter  should  be 
used,  and  the  following  table  gives  the  amounts  of  different  coloring  ma- 
terials required  to  produce  certain  shades : 

TABLE  OF  COLORS  TO  BE  USED  IN  PORTLAND  CEMENT 


COLOR  DESIRED 

COMMERCIAL  NAMES  OF  COLORS 
FOR  USB  IN  CEMENT 

APPROX- 
IMATE 
PRICES  PER 
LB.  IN  100- 
LB.  LOTS 
FOR  HIGH- 
GRADE 
COLORS 

POUNDS  OF  COLOR 
REQUIRED  FOR  EACH 
BAG  OF  CEMENT  TO 
SECURE  — 

Light 
Shade 

Medium 
Shade 

Grays,  blue-black  and  black 
Blue  shade  

f  Germantown  lampblack 
1  Carbon  black 
j  Black  oxide  of  manga- 
nese 
Ultramarine  blue 

Red  oxide  of  iron 
Mineral  turkey  red 

Indian  red 
Metallic  brown  (oxide) 

Yellow  ochre 
Chromium  oxide 

10  cts. 
8    " 

6    " 

18    " 

3    " 
15    " 

10    " 
4    " 

6    " 
26    " 

1A 

1A 

1 

5 

5 
5 

5 
5 

5 
5 

1 
1 

2 
10 

10 
10 

10 
10 

10 
10 

Brownish-red  to  dull  brick 
red             

Bright  red  to  vermilion  
Red  sandstone  to  purplish- 
red  

Brown  to  reddish-brown.  .  .  . 
Buff,  colonial  tint,  and  yel- 
low   

Green  shade                 

92 


2.  Form  Work. 

(a)  Construction  of  ornamental  molds  and  cores  for  flower-boxes, 
straight  line  vases,  round  fern  jars,  circular  vases,  with  and 
without  a  handle,  urns,  drinking  fountains,  bird  baths,  animal 
drinking  tanks,  etc. 

3.  Concrete  Work. 

(a)  Construction  of  exercises  selected  from  above.  (Note. — By 
this  time  the  students  should  be  sufficiently  familiar  with  the 
work  to  plan  original  exercises  of  their  own.  The  instructor 
should  encourage  originality,  and  also  give  the  students  oppor- 
tunity to  make  some  preferred  exercise.) 

EXERCISE  8 
THE  CONSTRUCTION  OF  A  CONCRETE  FLOWER-BOX 

(Plate  No.  14) 

The  long  window-box  shown  in  Plate  No.  14  should  be  made 
of  1  part  cement  and  two  parts  of  clean  sand,  and  will  take 
approximately  32  pounds  of  cement  and  ^  cubic  foot  of  sand. 
The  student  should  have  little  trouble  in  making  the  form  if 
the  directions  shown  on  the  plates  are  closely  followed. 

After  the  surfaces  of  the  form  that  are  to  come  in  contact  with 
the  concrete  have  been  well  oiled,  the  form  should  be  assembled. 
Reinforcing  should  consist  of  J^-inch  galvanized  square-mesh 
wire,  which  is  placed  in  position  by  making  a  basket  that, 
when  put  in  place  in  the  form,  will  be  }/%  inch  from  the  inside 
surface  of  the  form.  This  basket  can  be  held  in  place  by  slip- 
ping blocks  between  it  and  the  inside  form,  these  to  be  removed 
after  the  concrete  has  been  deposited  up  to  within  about  three 
inches  of  the  top  of  form. 

Sand  should  be  measured  out  accurately  and  placed  upon  the 
mixing  board  in  a  thin  layer.  The  cement  is  then  distributed 
over  the  sand  and  the  two  are  mixed  until  the  color  is  uniform. 
Water  should  be  added  slowly  from  a  sprinkling  can  until  the 
mixture  is  thin  enough  so  that  it  will  flow  readily  into  and  fill 
all  parts  of  the  form.  A  trowel,  or  a  thin,  flat  stick  with  a 
chisel  end,  should  be  worked  up  and  down  along  the  inside  of 
the  form  so  as  to  force  the  coarse  particles  of  the  mixture  away 
from  the  surface. 

It  is  not  advisable  to  remove  forms  until  the  flower-box  has 
been  in  the  mold  for  at  least  twenty-four  hours,  and  a  great  deal 
of  care  should  be  taken  when  removing  the  form  so  as  not  to 
damage  the  green  concrete.  After  removing  the  form  the  out- 
side surface  should  be  brushed  with  a  stiff  brush  or  an  old 
broom;  the  flower-box  should  then  be  allowed  to  air-dry  pos- 
sibly two  or  three  hours,  but  be  kept  from  drying  out  too  rap- 
idly; never  place  in  the  sunlight.  The  box  should  then  be 
placed  in  water  and  allowed  to  soak  for  three  or  four  days.  A 
93 


very  smooth  surface  can  be  obtained  by  sprinkling  dry  cement 
over  the  wet  surface  after  removing  from  the  water  and  rubbing 
the  cement  in  with  a  scrubbing-brush  or  with  a  block  of  cork. 

(The  above  directions  will  also  apply  hi  constructing  the  smaller 
flower-box  on  Plate  No.  14.) 


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